Add a new x86 specific instruction flag to force some isCodeGenOnly instructions to go through to the disassembler tables without resorting to string matches. Apply flag to all _REV instructions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198543 91177308-0d34-0410-b5e6-96231b3b80d8