[PATCH] Fix for PR18921.

[Stepan Dyatkovskiy]

  > I don't think the changes to the .td files are needed: basically there's no way for the parser to create a GPRPair operand so the definitions will never trigger.
  Yup, they likely won't. I'll do it then tomorrow.

  > ldrd/strd in Thumb mode accepts the offset up to 1020 (in multiples of 4)
  Is it what test/MC/ARM/ldrd-strd-gnu-arm-bad-imm.s checks? Anyway I'll recheck the reference.

  Thanks for review guys! I'll publish fix tomorrow. Its time to sleep now :-)

http://llvm-reviews.chandlerc.com/D3277