[NVPTX] allow address space inference for volatile loads/stores.

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