[dev.regabi] cmd/compile: cleanup for concrete types - more

Accumulated fixes to recent changes, to make the code safe
for automated deinterfacing.

Change-Id: I200737046cea88f3356b2402f09e2ca477fb8456
Reviewed-on: https://go-review.googlesource.com/c/go/+/279232


Trust: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>