If a labeled statement is the target of a goto, we must treat it as the
boundary of a new basic block, but only if it is not already the boundary
of a basic block such as a labeled for loop.

Fixes #16624

Now reports 100% coverage for the test in the issue.

Change-Id: If118bb6ff53a96c738e169d92c03cb3ce97bad0e
Reviewed-on: https://go-review.googlesource.com/30977Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>