commonPointerCast cleanups to make address space change easier

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188719 91177308-0d34-0410-b5e6-96231b3b80d8
Cleanup visitGetElementPtr to make address space change easier

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188720 91177308-0d34-0410-b5e6-96231b3b80d8
Teach InstCombine visitGetElementPtr about address spaces

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188721 91177308-0d34-0410-b5e6-96231b3b80d8