[ARM] Change the MC names for VMAXNM/VMINNM

Now the NEON ones have a prefix "NEON_", and the VFP ones have a
prefix "VFP_". This is so that the regex in ARMScheduleA57.td can be
made to match both of _those_ classes of VMAXNM without also matching
the MVE ones that are going to be introduced soon. NFCI.

Patch by Simon Tatham.

Differential Revision: https://reviews.llvm.org/D60700

llvm-svn: 362097