tree-optimization/102920 - fix PHI VN with undefined args

tree-optimization/102920 - fix PHI VN with undefined args

This fixes a latent issue exposed by now allowing VN_TOP in PHI
arguments.  We may only use optimistic equality when merging values on
different edges, not when merging values on the same edge - in particular
we may not choose the undef value on any edge when there's a not undef
value as well.

2021-10-25  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/102920
	* tree-ssa-sccvn.h (expressions_equal_p): Add argument
	controlling VN_TOP matching behavior.
	* tree-ssa-sccvn.c (expressions_equal_p): Likewise.
	(vn_phi_eq): Do not optimistically match VN_TOP.

	* gcc.dg/torture/pr102920.c: New testcase.