AMDGPU: Add cache invalidation instructions.

These are necessary for implementing mem_fence for
OpenCL 2.0.

The VI assembler tests are disabled since it seems to be
using the wrong encoding or opcode.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248532 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed