[ARM] Add support for unpredictable MVN instructions.

This fixes bugzilla 33011
https://bugs.llvm.org/show_bug.cgi?id=33011

Defines bits {19-16} as zero or unpredictable as specified by the ARM ARM in
sections A8.8.116 and A8.8.117.

It fixes also the usage of PC register as destination register for MVN
register-shifted register version as specified in A8.8.117.

Differential Revision: https://reviews.llvm.org/D41905

llvm-svn: 323954