gimple-fold: Improve optimize_memcpy_to_memset to handle STRING_CST [PR78408]

gimple-fold: Improve optimize_memcpy_to_memset to handle STRING_CST [PR78408]

While looking into PR 118947, I noticed that optimize_memcpy_to_memset didn't
handle STRING_CST which are also used for a memset of 0 but for char arrays.
This fixes that and improves optimize_memcpy_to_memset to handle that case.

This fixes part of PR 118947 but not the whole thing; we still need to skip over
vdefs in some cases.

Boostrapped and tested on x86_64-linux-gnu.

	PR tree-optimization/78408
	PR tree-optimization/118947

gcc/ChangeLog:

	* gimple-fold.cc (optimize_memcpy_to_memset): Handle STRING_CST case too.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr78408-3.c: New test.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>