Kill known equivalences before a new assignment in the path solver.

Every time we have a killing statement, we must also kill the relations
seen so far.  This is similar to what we did for the equivs inherent in
PHIs along a path.

Tested on x86-64 and ppc64le Linux.

gcc/ChangeLog:

	* gimple-range-path.cc
	(path_range_query::range_defined_in_block): Call killing_def.