The algorithm for placing a phi nodes in small functions now
unreachable. This patch fix that.

Change-Id: I253d745b414fa12ee0719459c28e78a69c6861ae
Reviewed-on: https://go-review.googlesource.com/30106
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>