cmd/compile: lvalues are only required for == when calling runtime fns

Fixes #18661.

Change-Id: I865802a9b88ab22560c9914a70901d1924242bdc
Reviewed-on: https://go-review.googlesource.com/35236
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

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

@@ -3117,12 +3117,12 @@ func walkcompare(n *Node, init *Nodes) *Node {
 		cmpr = cmpr.Left
 	}
 
-	if !islvalue(cmpl) || !islvalue(cmpr) {
-		Fatalf("arguments of comparison must be lvalues - %v %v", cmpl, cmpr)
-	}
-
 	// Chose not to inline. Call equality function directly.
 	if !inline {
+		if !islvalue(cmpl) || !islvalue(cmpr) {
+			Fatalf("arguments of comparison must be lvalues - %v %v", cmpl, cmpr)
+		}
+
 		// eq algs take pointers
 		pl := temp(ptrto(t))
 		al := nod(OAS, pl, nod(OADDR, cmpl, nil))

test/fixedbugs/issue18661.go

@@ -0,0 +1,18 @@
+// compile
+
+// Copyright 2017 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 p
+
+var (
+	e interface{}
+	s = struct{ a *int }{}
+	b = e == s
+)
+
+func test(obj interface{}) {
+	if obj != struct{ a *string }{} {
+	}
+}