cmd/compile: use ellipses in wasm rules

Also, explicitly zero AuxInt in some ops (like Div),
to make it clear why they do not use an ellipsis.

Passes toolstash-check -all.

Change-Id: I2294d10e47d904d03e489e6ca43d46679323f75d
Reviewed-on: https://go-review.googlesource.com/c/go/+/217009


Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>