[ARMv8] Add support for the NEON instructions vmaxnm/vminnm.

This adds a new class for non-predicable NEON instructions and a
new DecoderNamespace for v8 NEON instructions.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186504 91177308-0d34-0410-b5e6-96231b3b80d8