[release-branch.go1.21] cmd/compile: ensure empty blocks in write barriers are marked...

[release-branch.go1.21] cmd/compile: ensure empty blocks in write barriers are marked unpreemptible

Fixes #61958

Change-Id: I242ab77ad2f1ea1dad2d14ef756fa92f9378429f
Reviewed-on: https://go-review.googlesource.com/c/go/+/518755
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: David Chase <drchase@google.com>