Check in a patch that has already been code reviewed by Owen that I'd forgotten to commit.

Build on previous patches to successfully distinguish between an M-series and A/R-series MSR and MRS instruction. These take different mask names and have a *slightly* different opcode format.

Add decoder and disassembler tests.

Improvement on the previous patch - successfully distinguish between valid v6m and v7m masks (one is a subset of the other). The patch had to be edited slightly to apply to ToT.

llvm-svn: 140696