path relation oracle: Remove SSA's being killed from the equivalence list.

Same thing as the relational change.  Walk any equivalences that have
been registered on the path, and remove the name being killed.  The
only reason we had added the equivalence with itself earlier is so we
wouldn't search any further in the equivalency list.  So if we are
removing all references to it, then we no longer need to add a "kill"
record.

Will push pending tests on x86-64 Linux.

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

gcc/ChangeLog:

	* value-relation.cc (path_oracle::killing_def): Walk the
	equivalency list and remove SSA from any equivalencies.