[InstCombine] Fold phi ( inttoptr/ptrtoint x ) to phi (x)

The inttoptr/ptrtoint roundtrip optimization is not always correct.
We are working towards removing this optimization and adding support to specific cases where this optimization works.

In this patch, we focus on phi-node operands with inttoptr casts.
We know that ptrtoint( inttoptr( ptrtoint x) ) is same as ptrtoint (x).
So, we want to remove this roundtrip cast which goes through phi-node.

Reviewed By: aqjune

Differential Revision: https://reviews.llvm.org/D106289