tree-optimization/116906 - unsafe PRE with never executed edges

tree-optimization/116906 - unsafe PRE with never executed edges

When we're computing ANTIC for PRE we treat edges to not yet visited
blocks as having a maximum ANTIC solution to get at an optimistic
solution in the iteration.  That assumes the edges visted eventually
execute.  This is a wrong assumption that can lead to wrong code
(and not only non-optimality) when possibly trapping expressions
are involved as the testcases in the PR show.  The following mitigates
this by pruning trapping expressions from ANTIC computed when
maximum sets are involved.

	PR tree-optimization/116906
	* tree-ssa-pre.cc (prune_clobbered_mems): Add clean_traps
	argument.
	(compute_antic_aux): Direct prune_clobbered_mems to prune
	all traps when any MAX solution was involved in the ANTIC
	computation.
	(compute_partial_antic_aux): Adjust.

	* gcc.dg/pr116906-1.c: New testcase.
	* gcc.dg/pr116906-2.c: Likewise.