[AArch64] Fix registerAllocator assigns same register for base and wback in
pre/post-index load and store.

Patch by Steven Wu <stevenwu@apple.com>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@215390 91177308-0d34-0410-b5e6-96231b3b80d8