cmd/gc: cleanup SWITCH nodes after walk.

Keeping pointers from the pre-walk phase confuses
the race detection instrumentation.

Fixes #6418.

R=golang-dev, dvyukov, r
CC=golang-dev
https://golang.org/cl/13368057

src/cmd/gc/swt.c

@@ -820,6 +820,9 @@ walkswitch(Node *sw)
 		return;
 	}
 	exprswitch(sw);
+	// Discard old AST elements after a walk. They can confuse racewealk.
+	sw->ntest = nil;
+	sw->list = nil;
 }
 
 /*

src/pkg/runtime/race/testdata/mop_test.go

@@ -231,6 +231,22 @@ func TestRaceCaseFallthrough(t *testing.T) {
 	<-ch
 }
 
+func TestRaceCaseIssue6418(t *testing.T) {
+	m := map[string]map[string]string{
+		"a": map[string]string{
+			"b": "c",
+		},
+	}
+	ch := make(chan int)
+	go func() {
+		m["a"]["x"] = "y"
+		ch <- 1
+	}()
+	switch m["a"]["b"] {
+	}
+	<-ch
+}
+
 func TestRaceCaseType(t *testing.T) {
 	var x, y int
 	var i interface{} = x