cmd/compile: fix compilation crash with several blank labels

Fixes #52278

Change-Id: Ibf67c7b019feec277d316e04d93b458efea133fb
Reviewed-on: https://go-review.googlesource.com/c/go/+/399574
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/cmd/compile/internal/escape/escape.go

@@ -210,6 +210,9 @@ func (b *batch) walkFunc(fn *ir.Func) {
 		switch n.Op() {
 		case ir.OLABEL:
 			n := n.(*ir.LabelStmt)
+			if n.Label.IsBlank() {
+				break
+			}
 			if e.labels == nil {
 				e.labels = make(map[*types.Sym]labelState)
 			}

src/cmd/compile/internal/escape/stmt.go

@@ -50,6 +50,9 @@ func (e *escape) stmt(n ir.Node) {
 
 	case ir.OLABEL:
 		n := n.(*ir.LabelStmt)
+		if n.Label.IsBlank() {
+			break
+		}
 		switch e.labels[n.Label] {
 		case nonlooping:
 			if base.Flag.LowerM > 2 {

test/fixedbugs/issue52278.go

@@ -0,0 +1,12 @@
+// compile
+
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+func main() {
+_:
+_:
+}