[dev.ssa] cmd/compile/internal/ssa: add more critical edges

[dev.ssa] cmd/compile/internal/ssa: add more critical edges

Add blocks to remove critical edges, even when it looks like
there's no phi that requires it.  Regalloc still likes to have
critical-edge-free graphs for other reasons.

Change-Id: I69f8eaecbc5d79ab9f2a257c2e289d60b18e43c8
Reviewed-on: https://go-review.googlesource.com/13933
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>