[NewGVN] Add phi-of-ops operands if no real PHI is created.

If the PHI-of-ops simplifies to an existing value, no real PHI is
created, which means the dependencies between the
PHI-of-ops and its operands is not materialized in IR. At the
moment, we fail to create a real PHI node for the PHI-of-ops,
because the PHI-of-ops root instruction is not re-visited if
one of the PHI-of-ops operands changes. We need to add the
operands as additional users in this case.

Even with this patch, there are still some dependencies
missing. I will continue tackling the outstanding
reporeted crashes in this area.

Fixes PR36501, PR42422, PR42557.

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D66924