tree-optimization/111294 - better DCE after forwprop

tree-optimization/111294 - better DCE after forwprop

The following adds more aggressive DCE to forwprop to clean up dead
stmts when folding a stmt leaves some operands unused.  The patch
uses simple_dce_from_worklist for this purpose, queueing original
operands before substitution and folding, but only if we folded the
stmt.

This removes one dead stmt biasing threading costs in a later pass
but it doesn't resolve the optimization issue in the PR yet.

	PR tree-optimization/111294
	* tree-ssa-forwprop.cc (pass_forwprop::execute): Track
	operands that eventually become dead and use simple_dce_from_worklist
	to remove their definitions if they did so.

	* gcc.dg/tree-ssa/evrp10.c: Adjust.
	* gcc.dg/tree-ssa/evrp6.c: Likewise.
	* gcc.dg/tree-ssa/forwprop-31.c: Likewise.
	* gcc.dg/tree-ssa/neg-cast-3.c: Likewise.