Switch lowering splits each case expression out
into its own OCASE node.

Change-Id: Ifcb72b99975ed36da8540f6e43343e9aa2058572
Reviewed-on: https://go-review.googlesource.com/26769
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>