gc: optimize interface ==, !=

If the values being compared have different concrete types,
then they're clearly unequal without needing to invoke the
actual interface compare routine.  This speeds tests for
specific values, like if err == io.EOF, by about 3x.

benchmark                  old ns/op    new ns/op    delta
BenchmarkIfaceCmp100             843          287  -65.95%
BenchmarkIfaceCmpNil100          184          182   -1.09%

Fixes #2591.

R=ken2
CC=golang-dev
https://golang.org/cl/5651073

src/pkg/runtime/runtime_test.go

@@ -0,0 +1,40 @@
+// Copyright 2012 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 runtime_test
+
+import (
+	"io"
+	"testing"
+)
+
+var errf error
+
+func errfn() error {
+	return errf
+}
+
+func errfn1() error {
+	return io.EOF
+}
+
+func BenchmarkIfaceCmp100(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < 100; j++ {
+			if errfn() == io.EOF {
+				b.Fatal("bad comparison")
+			}
+		}
+	}
+}
+
+func BenchmarkIfaceCmpNil100(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for j := 0; j < 100; j++ {
+			if errfn1() == nil {
+				b.Fatal("bad comparison")
+			}
+		}
+	}
+}