[PATCH v2] X86: disambiguate unqualified btr, bts

[Ramkumar Ramachandra]

PaX Team wrote:
> http://llvm.org/bugs/show_bug.cgi?id=9362
>
> and the conclusion was to change the linux (user) side, not llvm.

I'm not understanding this: what is so special about btr/bts, and why
is the solution I'm proposing any different from 824a907 (which was
written a month before this bug was classified as WONTFIX), which
disambiguates bt as btl?  If llvm's policy is to reject all ambiguous
instructions, then 824a907 (and possibly other commits) must be
reverted for a consistency.

What is the problem with matching gas behavior?  Is there some insane
unpredictable magic going on in gas (can you give me inputs to
demonstrate this)?  If the instructions really are Wrong, why hasn't
linux.git been patched yet?  Can you write me a coherent explanation
as to why it is Wrong to support perfectly valid instruction
mnemonics, and how linux.git should be patched, in that case?

Thanks.