Add label and goto checks and improve test coverage.

Implement OSWITCH and OSELECT.

Implement OBREAK and OCONTINUE.

Allow generation of code in dead blocks.

Change-Id: Ibebb7c98b4b2344f46d38db7c9dce058c56beaac
Reviewed-on: https://go-review.googlesource.com/12445Reviewed-by: Keith Randall <khr@golang.org>