This re-enables functionality that inadvertently was disabled in the
(long) past.

Also, don't perform branch checks if we had errors in a function
to avoid spurious errors or (worst-case) crashes.

Slightly modified test/fixedbugs/issue14006.go to make sure the
test still reports invalid label errors (the surrounding function
must be syntactically correct).

Change-Id: Id5642930877d7cf3400649094ec75c753b5084b7
Reviewed-on: https://go-review.googlesource.com/69770
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>