cmd/compile: use ellipses in PPC64 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: I2e234e7b779422f7577037813f277e7586061880
Reviewed-on: https://go-review.googlesource.com/c/go/+/217012


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