tree-optimization/64731 - extend store-from CTOR lowering to TARGET_MEM_REF

tree-optimization/64731 - extend store-from CTOR lowering to TARGET_MEM_REF

The following also covers TARGET_MEM_REF when decomposing stores from
CTORs to supported elementwise operations.  This avoids spilling
and cleans up after vector lowering which doesn't touch loads or
stores.  It also mimics what we already do for loads.

	PR tree-optimization/64731
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Also
	handle TARGET_MEM_REF destinations of stores from vector
	CTORs.

	* gcc.target/i386/pr64731.c: New testcase.