cmd/compile: optimize generated struct/array equality code

Use a standard "not-equal" label that we can jump to when we
detect that the arguments are not equal. This prevents the
recombination that was noticed in #39428.

Fixes #39428

Change-Id: Ib7c6b3539f4f6046043fd7148f940fcdcab70427
Reviewed-on: https://go-review.googlesource.com/c/go/+/255317


Trust: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>