Fix wrong-code issue in VN

Fix wrong-code issue in VN

The following fixes the wrong removed dead store discovered on the
PR108657 testcase when the reported DSE issue is not fixed.
The issue is we were using ssa_undefined_value_p on virtual operands
which returns a result based on the real definition of the definition
statement.  That doesn't make sense so this patch guards the calls
properly and makes sure nobody else does the same mistake.

	* tree-ssa.cc (ssa_undefined_value_p): Assert we are not
	called on virtual operands.
	* tree-ssa-sccvn.cc (vn_phi_lookup): Guard
	ssa_undefined_value_p calls.
	(vn_phi_insert): Likewise.
	(set_ssa_val_to): Likewise.
	(visit_phi): Avoid extra work with equivalences for
	virtual operand PHIs.