forwprop: Move memcpy_to_memset from gimple fold to forwprop

forwprop: Move memcpy_to_memset from gimple fold to forwprop

Since this optimization now walks the vops, it is better to only
do it in forwprop rather than in all the time in fold_stmt.

The next patch will add the limit to the alias walk.

gcc/ChangeLog:

	* gimple-fold.cc (optimize_memcpy_to_memset): Move to
	tree-ssa-forwprop.cc.
	(gimple_fold_builtin_memory_op): Remove call to
	optimize_memcpy_to_memset.
	(fold_stmt_1): Likewise.
	* tree-ssa-forwprop.cc (optimize_memcpy_to_memset): Move from
	gimple-fold.cc.
	(simplify_builtin_call): Try to optimize memcpy/memset.
	(pass_forwprop::execute): Try to optimize memcpy like assignment
	from a previous memset.

gcc/testsuite/ChangeLog:

	* gcc.dg/pr78408-1.c: Update scan to forwprop1 only.

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