Use rtx_mode_t instead of std::make_pair

Use rtx_mode_t instead of std::make_pair

This change makes the code less sensitive to the exact type of the mode,
i.e. it forces a conversion where necessary.  This becomes important
when wrappers like scalar_int_mode and scalar_mode can also be used
instead of machine_mode.

Using rtx_mode_t also abstracts away the representation.  The fact that
it's a std::pair rather than a custom class isn't important to users of
the interface.

gcc/
2016-11-18  Richard Sandiford  <richard.sandiford@arm.com>
	    Alan Hayward  <alan.hayward@arm.com>
	    David Sherwood  <david.sherwood@arm.com>

	* combine.c (try_combine): Use rtx_mode_t instead of std::make_pair.
	* dwarf2out.c (mem_loc_descriptor, loc_descriptor): Likewise.
	(add_const_value_attribute): Likewise.
	* explow.c (plus_constant): Likewise.
	* expmed.c (expand_mult, make_tree): Likewise.
	* expr.c (convert_modes): Likewise.
	* loop-doloop.c (doloop_optimize): Likewise.
	* postreload.c (reload_cse_simplify_set): Likewise.
	* simplify-rtx.c (simplify_const_unary_operation): Likewise.
	(simplify_binary_operation_1, simplify_const_binary_operation):
	(simplify_const_relational_operation, simplify_immed_subreg): Likewise.
	* wide-int.h: Update documentation to recommend rtx_mode_t
	instead of std::make_pair.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@242586 138bc75d-0d04-0410-961f-82ee72b054a4