cmd/compile: make iface == iface const evaluation respect !=

Fixes #19911

Change-Id: Ib2b2505fe31ce00c6ffc021a0fe5df510633b44b
Reviewed-on: https://go-review.googlesource.com/40251
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

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

@@ -815,6 +815,9 @@ func evconst(n *Node) {
 	// check for compatible general types (numeric, string, etc)
 	if wl != wr {
 		if wl == TINTER || wr == TINTER {
+			if n.Op == ONE {
+				goto settrue
+			}
 			goto setfalse
 		}
 		goto illegal

test/fixedbugs/issue19911.go

+// run
+
+// 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 main
+
+import (
+	"fmt"
+	"strings"
+)
+
+type ET struct{}
+
+func (*ET) Error() string { return "err" }
+
+func main() {
+	check("false", fmt.Sprintf("(*ET)(nil) == error(nil): %v", (*ET)(nil) == error(nil)))
+	check("true", fmt.Sprintf("(*ET)(nil) != error(nil): %v", (*ET)(nil) != error(nil)))
+
+	nilET := (*ET)(nil)
+	nilError := error(nil)
+
+	check("false", fmt.Sprintf("nilET == nilError: %v", nilET == nilError))
+	check("true", fmt.Sprintf("nilET != nilError: %v", nilET != nilError))
+}
+
+func check(want, gotfull string) {
+	got := gotfull[strings.Index(gotfull, ": ")+len(": "):]
+	if got != want {
+		panic("want " + want + " got " + got + " from " + gotfull)
+	}
+}