cmd/compile: fix choice of phi building algorithm

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>

src/cmd/compile/internal/gc/phi.go

@@ -28,7 +28,7 @@ const debugPhi = false
 // of the appropriate phi or definition.
 // TODO: make this part of cmd/compile/internal/ssa somehow?
 func (s *state) insertPhis() {
-	if len(s.f.Blocks) <= smallBlocks && false {
+	if len(s.f.Blocks) <= smallBlocks {
 		sps := simplePhiState{s: s, f: s.f, defvars: s.defvars}
 		sps.insertPhis()
 		return