Do not walk equivalence set in path_oracle::killing_def.

When killing a def in the path ranger, there is no need to walk the set
of existing equivalences clearing bits.  An equivalence match requires
that both ssa-names have to be in each others set.  As killing_def
creates a new empty set contianing only the current def,  it already
ensures false equivaelnces won't happen.

	PR tree-optimization/106514
	* value-relation.cc (path_oracle::killing_def) Do not walk the
	  equivalence set clearing bits.