[NewGVN] Add ops as dependency if we cannot find a leader for ValueOp.

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 D43865
and this patch, bootstrapping clang/llvm works with -enable-newgvn, whereas
without it, the "value changed after iteration" assertion is triggered.

Reviewers: dberlin, davide

Reviewed By: dberlin

Differential Revision: https://reviews.llvm.org/D42180

llvm-svn: 330334