Add support for encoding the HLE XACQUIRE and XRELEASE prefixes.

For decoding, keep the current behavior of always decoding these as their REP
versions. In the future, this could be improved to recognize the cases where
these behave as XACQUIRE and XRELEASE and decode them as such.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184207 91177308-0d34-0410-b5e6-96231b3b80d8