arch-arm: implement VMINNM and VMAXNM scalar version

arch-arm: implement VMINNM and VMAXNM scalar version

ARMv8.2 16-bit versions have not yet been implemented, but a placeholders
were created for them.

Refactor the nearby decoding tree to closely match the ARM spec A32 decode
table.

That piece of the tree can also be called from thumb which decodes it in
the same way, although the thumb decode table has a different terminology

The old code didn't match neither A32 or T32 terminologies, so it is
better to at least match one of them to help verify correctness.

Change-Id: Iabbbca2932557cf6c98ce36690c385c3ddf39ed8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18690
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>