[SROA] Enhance SROA to handle `addrspacecast`ed allocas

Summary:
- After `addrspacecast` is allowed to be eliminated in SROA, the
  adjusting of storage pointer (from `alloca) needs to handle the
  potential different address spaces between the storage pointer (from
  alloca) and the pointer being used.

Reviewers: arsenm

Subscribers: wdng, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63501

llvm-svn: 363711