sra: Do not leave work for DSE (that it can sometimes not perform)

When looking again at the g++.dg/tree-ssa/pr109849.C testcase we
discovered that it generates terrible store-to-load forwarding stalls
because SRA was leaving behind aggregate loads but all the stores were
by scalar parts and DSE failed to remove the useless load.  SRA has
all the knowledge to remove the statement even now, so this small
patch makes it do so.

With this patch, the g++.dg/tree-ssa/pr109849.C micro-benchmark runs 9
times faster (on an AMD EPYC 75F3 machine).

gcc/ChangeLog:

2024-04-18  Martin Jambor  <mjambor@suse.cz>

	* tree-sra.cc (sra_modify_assign): Remove the original statement
	also when dealing with a store to a fully covered aggregate from a
	non-candidate.

gcc/testsuite/ChangeLog:

2024-04-23  Martin Jambor  <mjambor@suse.cz>

	* g++.dg/tree-ssa/pr109849.C: Also check that the aggeegate store
	to cur disappears.
	* gcc.dg/tree-ssa/ssa-dse-26.c: Instead of relying on DSE,
	check that the unwanted stores were removed at early SRA time.