tree-optimization/107569 - avoid wrecking earlier folding in FRE/PRE

The following avoids picking up dead code left over from folding
during FRE/PRE, effectively undoing propagations.

	PR tree-optimization/107569
	* tree-ssa-sccvn.cc (eliminate_dom_walker::eliminate_stmt):
	Do not push SSA names with zero uses as available leader.
	(process_bb): Likewise.

	* g++.dg/opt/pr107569.C: New testcase.