[dev.ssa] cmd/compile: short-circuit empty blocks

[dev.ssa] cmd/compile: short-circuit empty blocks

Empty blocks are introduced to remove critical edges.
After regalloc, we can remove any of the added blocks
that are still empty.

Change-Id: I0b40e95ac3a6cc1e632a479443479532b6c5ccd9
Reviewed-on: https://go-review.googlesource.com/18833
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>