tree-optimization/79333 - fold stmts following SSA edges in VN

tree-optimization/79333 - fold stmts following SSA edges in VN

This makes sure to follow SSA edges when folding eliminated stmts.
This reaps the same benefit as forwprop folding all stmts, not
waiting for one to produce copysign in the new testcase.

2021-05-04  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/79333
	* tree-ssa-sccvn.c (eliminate_dom_walker::eliminate_stmt):
	Fold stmt following SSA edges.

	* gcc.dg/tree-ssa/ssa-fre-94.c: New testcase.
	* gcc.dg/graphite/fuse-1.c: Adjust.
	* gcc.dg/pr43864-4.c: Likewise.