cmd/compile: ignore div/mod in prove on non-x86 architectures

cmd/compile: ignore div/mod in prove on non-x86 architectures

Instead of writing AuxInt during prove and then zeroing it during lower,
just don't write it in the first place.

Passes toolstash-check -all.

Change-Id: Iea4b555029a9d69332e835536f9cf3a42b8223db
Reviewed-on: https://go-review.googlesource.com/c/go/+/220682
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>