tree-sra: Do not create stores into const aggregates (PR111873)

tree-sra: Do not create stores into const aggregates (PR111873)

This patch fixes (hopefully the) one remaining place where gimple SRA
was still creating a load into const aggregates.  It occurs when there
is a replacement for a load but that replacement is not type
compatible - typically because it is a single field structure.

I have used testcases from duplicates because the original test-case
no longer reproduces for me.

gcc/ChangeLog:

2025-05-13  Martin Jambor  <mjambor@suse.cz>

	PR tree-optimization/111873
	* tree-sra.cc (sra_modify_expr): When processing a load which has
	a type-incompatible replacement, do not store the contents of the
	replacement into the original aggregate when that aggregate is
	const.

gcc/testsuite/ChangeLog:

2025-05-13  Martin Jambor  <mjambor@suse.cz>

	* gcc.dg/ipa/pr120044-1.c: New test.
	* gcc.dg/ipa/pr120044-2.c: Likewise.
	* gcc.dg/tree-ssa/pr114864.c: Likewise.