cmd/compile: use typed aux in arm64 MOVstore rules

Introduces a few casts, mostly to fix rules that mix int64 and int32
off1 and off2.

Passes

  GOARCH=arm64 gotip build -toolexec 'toolstash -cmp' -a std

Change-Id: I1ec75211f3bb8e521dcc5217cf29ab0655a84d79
Reviewed-on: https://go-review.googlesource.com/c/go/+/230840
Run-TryBot: Alberto Donizetti <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: ALTree