cmd/gc: avoid creating circular lists when compiling with race detector.

Fixes #5431.

R=dvyukov, remyoudompheng, rsc
CC=gobot, golang-dev
https://golang.org/cl/9910043

src/cmd/gc/racewalk.c

@@ -255,7 +255,11 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 		// side effects are safe.
 		// n->right may not be executed,
 		// so instrumentation goes to n->right->ninit, not init.
-		l = nil;
+		// If right->ninit is non-nil, racewalknode might append it to itself.
+		// nil it out and handle it separately before putting it back.
+		l = n->right->ninit;
+		n->right->ninit = nil;
+		racewalklist(l, nil);
 		racewalknode(&n->right, &l, wr, 0);
 		appendinit(&n->right, l);
 		goto ret;

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

@@ -160,3 +160,18 @@ func noRaceReturn(c chan int) (a, b int) {
 	}()
 	return a, 10
 }
+
+func issue5431() {
+	var p **inltype
+	if inlinetest(p).x && inlinetest(p).y {
+	} else if inlinetest(p).x || inlinetest(p).y {
+	}
+}
+
+type inltype struct {
+	x, y bool
+}
+
+func inlinetest(p **inltype) *inltype {
+	return *p
+}