[InstCombine] Directly replace instr in foldIntegerTypedPHI() (NFCI)

Rather than inserting a ptrtoint + inttoptr pair, directly replace
the inttoptr with the new phi node. This ensures that no other
transform can undo it before the pair gets folded away.

This avoids the infinite loop when combined with D134954.

This is NFCI in the sense that it shouldn't make a difference, but
could due to different worklist order.