[dev.regabi] cmd/compile: cleanup for concrete types - range, select, swt

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 range.go, select.go, and swt.go: the big
control structures.

Passes buildall w/ toolstash -cmp.

Change-Id: I033fe056a7b815edb6e8a06f45c12ffd990f4d45
Reviewed-on: https://go-review.googlesource.com/c/go/+/277929


Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>