Remove poly_int_pod

Remove poly_int_pod

poly_int was written before the switch to C++11 and so couldn't
use explicit default constructors.  This led to an awkward split
between poly_int_pod and poly_int.  poly_int simply inherited from
poly_int_pod and added constructors, with the argumentless constructor
having an empty body.  But inheritance meant that poly_int had to
repeat the assignment operators from poly_int_pod (again, no C++11,
so no "using" to inherit base-class implementations).

All that goes away if we switch to using default constructors.

The main complication is ensuring that braced initialisation still
gives a constexpr, so that static variables can be initialised without
runtime code.  The two problems here are:

(1) When initialising a poly_int<N, wide_int> with fewer than N
    coefficients, the other coefficients need to be a zero of
    the same precision as the explicit coefficients.  This was
    previously done in a for loop using wi::ints_for<...>::zero,
    but C++11 constexpr constructors can't have function bodies.
    The patch instead uses a series of delegated initialisers to
    fill in the implicit coefficients.

(2) The initialisation in:

      void f(int x) {
        unsigned int foo {x};
      }

    produces the warning:

      warning: narrowing conversion of 'x' from 'int' to 'unsigned int' [-Wnarrowing]

    whereas:

      void f(int x) {
        unsigned int foo = x;
      }

    does not.  So switching to direct initialisation of the coeffs array
    would mean that:

      poly_uin64_t x = 0;

    would trigger a warning for using 0 rather than 0u.  That seemed
    overly pedantic, so the patch adds explicit casts to the constructor.
    The complication is to do that without adding extra code to
    wide-int versions.  The patch uses a new init_cast type for that.

gcc/
	* poly-int.h (poly_int_pod): Delete.
	(poly_coeff_traits::init_cast): New type.
	(poly_int_full, poly_int_hungry, poly_int_fullness): New structures.
	(poly_int): Replace constructors that take 1 and 2 coefficients with
	a general one that takes an arbitrary number of coefficients.
	Delegate initialization to two new private constructors, one of
	which uses the coefficients as-is and one of which adds an extra
	zero of the appropriate type (and precision, where applicable).
	(gt_ggc_mx, gt_pch_nx): Operate on poly_ints rather than poly_int_pods.
	* poly-int-types.h (poly_uint16_pod, poly_int64_pod, poly_uint64_pod)
	(poly_offset_int_pod, poly_wide_int_pod, poly_widest_int_pod): Delete.
	* gengtype.cc (main): Don't register poly_int64_pod.
	* calls.cc (initialize_argument_information): Use poly_int rather
	than poly_int_pod.
	(combine_pending_stack_adjustment_and_call): Likewise.
	* config/aarch64/aarch64.cc (pure_scalable_type_info): Likewise.
	* data-streamer.h (bp_unpack_poly_value): Likewise.
	* dwarf2cfi.cc (struct dw_trace_info): Likewise.
	(struct queued_reg_save): Likewise.
	* dwarf2out.h (struct dw_cfa_location): Likewise.
	* emit-rtl.h (struct incoming_args): Likewise.
	(struct rtl_data): Likewise.
	* expr.cc (get_bit_range): Likewise.
	(get_inner_reference): Likewise.
	* expr.h (get_bit_range): Likewise.
	* fold-const.cc (split_address_to_core_and_offset): Likewise.
	(ptr_difference_const): Likewise.
	* fold-const.h (ptr_difference_const): Likewise.
	* function.cc (try_fit_stack_local): Likewise.
	(instantiate_new_reg): Likewise.
	* function.h (struct expr_status): Likewise.
	(struct args_size): Likewise.
	* genmodes.cc (ZERO_COEFFS): Likewise.
	(mode_size_inline): Likewise.
	(mode_nunits_inline): Likewise.
	(emit_mode_precision): Likewise.
	(emit_mode_size): Likewise.
	(emit_mode_nunits): Likewise.
	* gimple-fold.cc (get_base_constructor): Likewise.
	* gimple-ssa-store-merging.cc (struct symbolic_number): Likewise.
	* inchash.h (class hash): Likewise.
	* ipa-modref-tree.cc (modref_access_node::dump): Likewise.
	* ipa-modref.cc (modref_access_analysis::merge_call_side_effects):
	Likewise.
	* ira-int.h (ira_spilled_reg_stack_slot): Likewise.
	* lra-eliminations.cc (self_elim_offsets): Likewise.
	* machmode.h (mode_size, mode_precision, mode_nunits): Likewise.
	* omp-low.cc (omplow_simd_context): Likewise.
	* pretty-print.cc (pp_wide_integer): Likewise.
	* pretty-print.h (pp_wide_integer): Likewise.
	* reload.cc (struct decomposition): Likewise.
	* reload.h (struct reload): Likewise.
	* reload1.cc (spill_stack_slot_width): Likewise.
	(struct elim_table): Likewise.
	(offsets_at): Likewise.
	(init_eliminable_invariants): Likewise.
	* rtl.h (union rtunion): Likewise.
	(poly_int_rtx_p): Likewise.
	(strip_offset): Likewise.
	(strip_offset_and_add): Likewise.
	* rtlanal.cc (strip_offset): Likewise.
	* tree-dfa.cc (get_ref_base_and_extent): Likewise.
	(get_addr_base_and_unit_offset_1): Likewise.
	(get_addr_base_and_unit_offset): Likewise.
	* tree-dfa.h (get_ref_base_and_extent): Likewise.
	(get_addr_base_and_unit_offset_1): Likewise.
	(get_addr_base_and_unit_offset): Likewise.
	* tree-ssa-loop-ivopts.cc (struct iv_use): Likewise.
	(strip_offset): Likewise.
	* tree-ssa-sccvn.h (struct vn_reference_op_struct): Likewise.
	* tree.cc (ptrdiff_tree_p): Likewise.
	* tree.h (poly_int_tree_p): Likewise.
	(ptrdiff_tree_p): Likewise.
	(get_inner_reference): Likewise.

gcc/testsuite/
	* gcc.dg/plugin/poly-int-tests.h (test_num_coeffs_extra): Use
	poly_int rather than poly_int_pod.