[SROA] Teach SROA to perform no-op pointer conversion.

Summary:
- When promoting a pointer from memory to register, SROA skips pointers
  from different address spaces. However, as `ptrtoint` and `inttoptr`
  are defined as no-op casts if that integer type has the same as the
  pointer value, generate the pair of `ptrtoint`/`inttoptr` (no-op cast)
  sequence to convert pointers from different address spaces if they
  have the same size.

Reviewers: arsenm, chandlerc, lebedev.ri

Subscribers:

Differential Revision: https://reviews.llvm.org/D81943