[ValueTracking] Look through constant Int2Ptr/Ptr2Int expressions

Summary:
This is analogous to the int2ptr/ptr2int instruction handling introduced
in D54956.

Reviewers: fhahn, efriedma, spatel, nlopes, sanjoy, lebedev.ri

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64708

llvm-svn: 366036