[release-branch.go1.8] cmd/compile: repaired loop-finder to handle trickier nesting

[release-branch.go1.8] cmd/compile: repaired loop-finder to handle trickier nesting

The loop-A-encloses-loop-C code did not properly handle the
case where really C was already known to be enclosed by B,
and A was nearest-outer to B, not C.

Fixes #19217.

Change-Id: I755dd768e823cb707abdc5302fed39c11cdb34d4
Reviewed-on: https://go-review.googlesource.com/39596
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>