tree-optimization/118895 - ICE during PRE

tree-optimization/118895 - ICE during PRE

When we simplify a NARY during PHI translation we have to make sure
to not inject not available operands into it given that might violate
the valueization hook constraints and we'd pick up invalid
context-sensitive data in further simplification or as in this case
later ICE when we try to insert the expression.

	PR tree-optimization/118895
	* tree-ssa-sccvn.cc (vn_nary_build_or_lookup_1): Only allow
	CSE if we can verify the result is available.

	* gcc.dg/pr118895.c: New testcase.