[PATCH v2] X86: disambiguate unqualified btr, bts

[Tim Northover]

> Cool. Sounds like we have a winner, then. Default to ‘l’ and issue a
> diagnostic for immediates outside the range [0,31].

I think that solution only really makes consistent sense if you forget
about "btw", which has the same problem for immediates above 15. To me
it seems like it's taking half a stand that the immediate shouldn't
wrap, but not a full one.

For what it's worth, I'd favour a btl/btq choice combined with an
error for > 63 (we don't need to include btw, since btl is more
compact and does the same job; it's just there to mess with our
plans). It avoids possibly decidedly non-obvious memory operand
fiddling but does what most people would expect for as long as
possible.

Also, if we're inventing syntax like this we should probably document
it somewhere, regardless of what we choose.

Cheers.

Tim.