If particular target supports volatile memory access operations, we can
avoid AS casting to generic AS. Currently it's only enabled in NVPTX for
loads and stores that access global & shared AS.
Differential Revision: https://reviews.llvm.org/D39026
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316495 91177308-0d34-0410-b5e6-96231b3b80d8