[InferAddressSpaces] Handle the pair of `ptrtoint`/`inttoptr`.

Summary:
- `ptrtoint` and `inttoptr` are defined as no-op casts if the integer
  value as the same size as the pointer value. The pair of
  `ptrtoint`/`inttoptr` is in fact a no-op cast sequence between
  different address spaces. Teach `infer-address-spaces` to handle them
  like a `bitcast`.

Reviewers: arsenm, chandlerc

Subscribers: jvesely, wdng, nhaehnle, hiraditya, kerbowa, cfe-commits, llvm-commits

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D81938