Fix for PR18921 (LDRD/STRD part)::
Removed "GNU Assembler extension (compatibility)" definitions from ARMInstrInfo.td
Fixed ARMAsmParser::ParseInstruction GNU compatability branch, so it also works for thumb mode from now.
Added new tests.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@205622 91177308-0d34-0410-b5e6-96231b3b80d8