[IRCE] Drop SCEV of a Phi after adding a new input. PR57335

Since SCEV learned to look through single value phis with
20d798bd, whenever we add
a new input to a Phi, we should make sure that the old cached
value is dropped. Otherwise, it may lead to various miscompiles,
such as breach of dominance as shown in the bug
https://github.com/llvm/llvm-project/issues/57335