cmd/compile: check domination of loop return in both controls

cmd/compile: check domination of loop return in both controls

Fixes #74473

Change-Id: I72ff6b95955ae9407271508aa80f230dcf1b6c74
Reviewed-on: https://go-review.googlesource.com/c/go/+/685816
Reviewed-by: Mark Freeman <mark@golang.org>
Auto-Submit: Jorropo <jorropo.pgm@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Keith Randall <khr@golang.org>