Thumb2 M-class MSR instruction support changes

This patch implements a few changes related to the Thumb2 M-class MSR instruction:
 * better handling of unpredictable encodings,
 * recognition of the _g and _nzcvqg variants by the asm parser only if the DSP
   extension is available, preferred output of MSR APSR moves with the _<bits>
   suffix for v7-M.

Patch by Petr Pavlu.

llvm-svn: 216874