[PATCH] Fix for PR18921.

[Stepan Dyatkovskiy]

  As summary (sorry, I'm still new with it):
  Fix for PR18921:
  Added GNU Assembler compatiblity definitions for post/pre increments in ARMInstrInfo.td.
  Added GNU Assembler compatiblity definitions for thumb2: ARMInstrThumb2.td.
  Fixed ARMAsmParser::ParseInstruction GNU compatability branch, so it also works for thumb mode from now.

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