cmd/compile: rewrite Wasm rules to use typed aux fields

Passes toolstash-check -all.

Change-Id: Ib731a59eadfffa81914848005b0f757649affa6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/228819
Run-TryBot: Richard Musiol <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: neelance