MC/X86: Chris pointed that 'as' isn't consistent in accepting the long form of

instructions which have no direct register usage.

Darwin 'as' accepts:
  add $0, (%rax)
but rejects
  mov $0, (%rax)
for example.

Given that, only accept suffix matches which match exactly one form. We still
need to emit nice diagnostics for failures...

llvm-svn: 103015