cmd/compile: add "loop-transformed" (for whole loop) to logopt

cmd/compile: add "loop-transformed" (for whole loop) to logopt

This is intended to support automated pairing of performance
regressions with transformed loops; there is already a POC
for doing this in the general missed-optimization case; the
difference here is the ability to describe an entire range,
which required some extra plumbing to acquire and publish
the ending line+column.

Change-Id: Ibe606786f6be917b5a9a69d773560ed716a0754d
Reviewed-on: https://go-review.googlesource.com/c/go/+/492717
Run-TryBot: David Chase <drchase@google.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>