Decorating vptr load & stores with !invariant.group

Adding !invariant.group to vptr load/stores for devirtualization purposes.
For more goto:
http://lists.llvm.org/pipermail/cfe-dev/2015-July/044227.html

http://reviews.llvm.org/D12026

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@247725 91177308-0d34-0410-b5e6-96231b3b80d8