go.tools/ssa: opt: improve the "unused φ-node" check to eliminate self-refs, debug-refs.

This removes about 5% of φ-nodes in one large program
and eliminates many zero-value constants.

(This does cause some Idents to no longer map to an ssa.Value.
This is observable in the oracle, whose tests are here updated.)

R=gri, gri
CC=golang-dev
https://golang.org/cl/26980043

oracle/testdata/src/main/describe-json.go

@@ -15,7 +15,7 @@ func main() { //
 	if i == nil {
 		i = new(D)
 	}
-	_ = i // @describe desc-val-i "i"
+	print(i) // @describe desc-val-i "\\bi\\b"
 
 	go main() // @describe desc-stmt "go"
 }

oracle/testdata/src/main/describe-json.golden

@@ -98,7 +98,7 @@
 	"mode": "describe",
 	"describe": {
 		"desc": "identifier",
-		"pos": "testdata/src/main/describe-json.go:18:6",
+		"pos": "testdata/src/main/describe-json.go:18:8",
 		"detail": "value",
 		"value": {
 			"type": "I",

oracle/testdata/src/main/describe.go

@@ -45,7 +45,7 @@ func main() { // @describe func-def-main "main"
 	if i != nil {
 		i = D{} // @describe var-ref-i-D "i"
 	}
-	_ = i // @describe var-ref-i "i"
+	print(i) // @describe var-ref-i "\\bi\\b"
 
 	// const objects
 	const localpi = 3.141     // @describe const-local-pi "localpi"

ssa/lift.go

@@ -209,8 +209,8 @@ func lift(fn *Function) {
 		nps := newPhis[b]
 		j := 0
 		for _, np := range nps {
-			if len(*np.phi.Referrers()) == 0 {
-				continue // unreferenced phi
+			if !phiIsLive(np.phi) {
+				continue // discard it
 			}
 			nps[j] = np
 			j++
@@ -266,6 +266,19 @@ func lift(fn *Function) {
 	fn.Locals = fn.Locals[:j]
 }
 
+func phiIsLive(phi *Phi) bool {
+	for _, instr := range *phi.Referrers() {
+		if instr == phi {
+			continue // self-refs don't count
+		}
+		if _, ok := instr.(*DebugRef); ok {
+			continue // debug refs don't count
+		}
+		return true
+	}
+	return false
+}
+
 type blockSet struct{ big.Int } // (inherit methods from Int)
 
 // add adds b to the set and returns true if the set changed.