This is patch [3/4] in a series to add assembler/disassembler support for
SVE's contiguous LD1 (scalar+scalar) instructions:
Minor clean-up of patch:
- Removed template argument from 'printRegWithShiftExtend'
- Added unused attribute to DecodeGPR64commonRegisterClass (will be removed in next patch, but avoids breaking buildbot).
- Rephrased diagnostic to say 'xzr' instead of 'x31'.