x86 -- disassemble the REP/REPNE prefix when needed

This fixes Apple bug: 13493622

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177887 91177308-0d34-0410-b5e6-96231b3b80d8
x86 -- add the XTEST instruction

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177888 91177308-0d34-0410-b5e6-96231b3b80d8