Kill second order relations in the path solver.

Kill second order relations in the path solver.

My upcoming work replacing the VRP threaders with a fully resolving
backward threader has tripped over various corner cases in the path
sensitive relation oracle.  This patch kills second order relations when
we kill a relation.

Tested on x86-64 and ppc64le Linux.

Co-authored-by: Andrew MacLeod <amacleod@redhat.com>
gcc/ChangeLog:

	* value-relation.cc (path_oracle::killing_def): Kill second
	order relations.