[ARM][MVE] Decoding of VMSR doesn't diagnose some unpredictable encodings

Decoding of VMSR doesn't diagnose some unpredictable encodings, as the unpredictable bits are not correctly set.

Diff-reduce this instruction's internals WRT VMRS so I can see the differences better. Mostly this is s/src/Rt/g.

Fill in the "should-be-(0)" bits.

Designate the Unpredictable{} bits for both VMRS and VMSR.

Patch by Mark Murray!

Differential revision: https://reviews.llvm.org/D66938

llvm-svn: 370729