Extend store merging to STRING_CST

Extend store merging to STRING_CST

The GIMPLE store merging pass doesn't merge STRING_CSTs in the general
case, although they are accepted by native_encode_expr; the reason is
that the pass only works with integral modes, i.e. with chunks whose
size is a power of two.

There are two possible ways of extending it to handle STRING_CSTs:
1) lift the condition of integral modes and treat STRING_CSTs as
other _CST nodes but with arbitrary size; 2) implement a specific
and separate handling for STRING_CSTs.

The attached patch implements 2) for the following reasons: on the
one hand, even in Ada where character strings are first-class citizens,
cases where merging STRING_CSTs with other *_CST nodes would be possible
are quite rare in practice; on the other hand, string concatenations
happen more naturally and frequently thanks to the "&" operator, giving
rise to merging opportunities.

gcc/ChangeLog:
	* gimple-fold.c (gimple_fold_builtin_memory_op): Fold calls that
	were initially created for the assignment of a variable-sized
	object and whose source is now a string constant.
	* gimple-ssa-store-merging.c (struct merged_store_group): Document
	STRING_CST for rhs_code field.
	Add string_concatenation boolean field.
	(merged_store_group::merged_store_group): Initialize it as well as
	bit_insertion here.
	(merged_store_group::do_merge): Set it upon seeing a STRING_CST.
	Also set bit_insertion here upon seeing a BIT_INSERT_EXPR.
	(merged_store_group::apply_stores): Clear it for small regions.
	Do not create a power-of-2-sized buffer if it is still true.
	And do not set bit_insertion here again.
	(encode_tree_to_bitpos): Deal with BLKmode for the expression.
	(merged_store_group::can_be_merged_into): Deal with STRING_CST.
	(imm_store_chain_info::coalesce_immediate_stores): Set bit_insertion
	to true after changing MEM_REF stores into BIT_INSERT_EXPR stores.
	(count_multiple_uses): Return 0 for STRING_CST.
	(split_group): Do not split the group for a string concatenation.
	(imm_store_chain_info::output_merged_store): Constify and rename
	some local variables.  Build an array type as destination type
	for a string concatenation, as well as a zero mask, and call
	build_string to build the source.
	(lhs_valid_for_store_merging_p): Return true for VIEW_CONVERT_EXPR.
	(pass_store_merging::process_store): Accept STRING_CST on the RHS.
	* gimple.h (gimple_call_alloca_for_var_p): New accessor function.
	* gimplify.c (gimplify_modify_expr_to_memcpy): Set alloca_for_var.
	* tree.h (CALL_ALLOCA_FOR_VAR_P): Document it for BUILT_IN_MEMCPY.

gcc/testsuite/ChangeLog:
	* gnat.dg/opt87.adb: New test.
	* gnat.dg/opt87_pkg.ads: New helper.
	* gnat.dg/opt87_pkg.adb: Likewise.