cmd/compile: add dedicated ARM64BitField aux type

cmd/compile: add dedicated ARM64BitField aux type

The goal here is improved AuxInt printing in ssa.html.
Instead of displaying an inscrutable encoded integer,
it displays something like

v25 (28) = UBFX <int> [lsb=4,width=8] v52

which is much nicer for debugging.

Change-Id: I40713ff7f4a857c4557486cdf73c2dff137511ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/221420
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>