tree-optimization/115149 - VOP live and missing PHIs

tree-optimization/115149 - VOP live and missing PHIs

The following fixes a bug in vop-live get_live_in which was using
NULL to indicate the first processed edge but at the same time
using it for the case the live-in virtual operand cannot be computed.
The following fixes this, avoiding sinking a load to a place where
we'd have to insert virtual PHIs to make the virtual operand SSA
web OK.

	PR tree-optimization/115149
	* tree-ssa-live.cc (virtual_operand_live::get_live_in):
	Explicitly track the first processed edge.

	* gcc.dg/pr115149.c: New testcase.