Add an x86 prefix encoding for instructions that would decode to a different instruction with 0xf2/f3/66 were in front of them, but don't themselves have a prefix. For now this doesn't change any bbehavior, but plan to use it to fix some bugs in the disassembler.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@201538 91177308-0d34-0410-b5e6-96231b3b80d8