MC/X86: Add "support" for matching ATT style mnemonic prefixes.

MC/X86: Add "support" for matching ATT style mnemonic prefixes.
 - The idea is that when a match fails, we just try to match each of +'b', +'w',
   +'l'. If exactly one matches, we assume this is a mnemonic prefix and accept
   it. If all match, we assume it is width generic, and take the 'l' form.

 - This would be a horrible hack, if it weren't so simple. Therefore it is an
   elegant solution! Chris gets the credit for this particular elegant
   solution. :)

 - Next step to making this more robust is to have the X86 matcher generate the
   mnemonic prefix information. Ideally we would also compute up-front exactly
   which mnemonic to attempt to match, but this may require more custom code in
   the matcher than is really worth it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103012 91177308-0d34-0410-b5e6-96231b3b80d8