tree-optimization/117965 - phiprop validity checking is too strict

tree-optimization/117965 - phiprop validity checking is too strict

The PR shows that when using std::clamp from the C++ standard library
and there is surrounding code using exceptions then phiprop can fail
to simplify the code so phiopt can turn the clamping into efficient
min/max operations.

The validation code is needlessly complicated, steming from the
time we had memory-SSA with multiple virtual operands.  The following
simplifies this, thereby fixing this issue.

	PR tree-optimization/117965
	* tree-ssa-phiprop.cc (phivn_valid_p): Remove.
	(propagate_with_phi): Pass in virtual PHI node from BB,
	rewrite load motion validity check to require the same
	virtual use along all paths.

	* g++.dg/tree-ssa/pr117965-1.C: New testcase.
	* g++.dg/tree-ssa/pr117965-2.C: Likewise.