If those operands change, we might find a leader for ValueOp, which
could enable new phi-of-op creation.
This fixes a case where we missed creating a phi-of-ops node. With
this patch, bootstrapping clang/llvm works with -enable-newgvn, whereas
without it, the "value changed after iteration" assertion is triggered.
This is based on @dberlin's patch shared for D43865, because it is
slightly clearer what's going on, but it does not really depend on it.