If both branches of a write barrier test go to the same block,
then there's no unsafe points.

This can only happen if the resulting memory state is somehow dead,
which can only occur in degenerate cases, like infinite loops. No
point in cleaning up the useless branch in these situations.

Fixes #26024.

Change-Id: I93a7df9fdf2fc94c6c4b1fe61180dc4fd4a0871f
Reviewed-on: https://go-review.googlesource.com/123655Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>