MS ABI: Emit more canonical vbptr stores and loads

This eliminates some i8* GEPs and makes the IR that clang emits a bit
more canonical. More work is needed for vftables, but that isn't a clear
win so I plan to send it for review.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220398 91177308-0d34-0410-b5e6-96231b3b80d8