[dev.unified] cmd/compile/internal/walk: minor prep refactoring

[dev.unified] cmd/compile/internal/walk: minor prep refactoring

Two small refactorings that will make it easier to thread through
RType parameters later. Behavior preserving, but seemed worth
separating out.

Passes toolstash -cmp.

Change-Id: I77905775015b6582bad2b32dd7700880c415893f
Reviewed-on: https://go-review.googlesource.com/c/go/+/413354
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>