Tweak ARM assembly parsing and printing of MSR instruction.

Tweak ARM assembly parsing and printing of MSR instruction.

The system register spec should be case insensitive. The preferred form for
output with mask values of 4, 8, and 12 references APSR rather than CPSR.
Update and tidy up tests accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@135532 91177308-0d34-0410-b5e6-96231b3b80d8