Add CLAC/STAC instruction encoding/decoding support

As these two instructions in AVX extension are privileged instructions for
special purpose, it's only expected to be used in inlined assembly.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179266 91177308-0d34-0410-b5e6-96231b3b80d8