[dev.regabi] cmd/compile: remove unnecessary String methods

There were only a few places these were still used, none of which
justify generating all this code. Instead rewrite them to use
fmt.Sprint or simpler means.

Passes buildall w/ toolstash -cmp.

Change-Id: Ibd123a1696941a597f0cb4dcc96cda8ced672140
Reviewed-on: https://go-review.googlesource.com/c/go/+/276072


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