[x86] Disambiguate RET[QL] and fix aliases for 16-bit mode

[x86] Disambiguate RET[QL] and fix aliases for 16-bit mode

I couldn't see how to do this sanely without splitting RETQ from RETL.

Eric says: "sad about the inability to roundtrip them now, but...".
I have no idea what that means, but perhaps it wants preserving in the
commit comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198756 91177308-0d34-0410-b5e6-96231b3b80d8