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

An automated rewrite will add concrete type assertions after
a test of n.Op(), when n can be safely type-asserted
(meaning, n is not reassigned a different type, n is not reassigned
and then used outside the scope of the type assertion,
and so on).

This sequence of CLs handles the code that the automated
rewrite does not: adding specific types to function arguments,
adjusting code not to call n.Left() etc when n may have multiple
representations, and so on.

This CL focuses on walk.go.

Passes buildall w/ toolstash -cmp.

Change-Id: I7aab57e4077cf10da1994625575c5e42ad114a9c
Reviewed-on: https://go-review.googlesource.com/c/go/+/277921


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>