bytes: add benchmark for LastIndex

Add BenchmarkLastIndexHard[1-3] in preparation for implementing
LastIndex using Rabin-Karp akin to strings.LastIndex

BenchmarkLastIndexHard1-8   	     500	   3162694 ns/op
BenchmarkLastIndexHard2-8   	     500	   3170475 ns/op
BenchmarkLastIndexHard3-8   	     500	   3051127 ns/op

Change-Id: Id99f85f9640e248958f2b4be4dfd8c974e3b50e7
Reviewed-on: https://go-review.googlesource.com/c/go/+/166257
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/bytes/bytes_test.go

@@ -1642,6 +1642,16 @@ func makeBenchInputHard() []byte {
 
 var benchInputHard = makeBenchInputHard()
 
+func benchmarkLastIndexHard(b *testing.B, sep []byte) {
+	for i := 0; i < b.N; i++ {
+		LastIndex(benchInputHard, sep)
+	}
+}
+
+func BenchmarkLastIndexHard1(b *testing.B) { benchmarkLastIndexHard(b, []byte("<>")) }
+func BenchmarkLastIndexHard2(b *testing.B) { benchmarkLastIndexHard(b, []byte("</pre>")) }
+func BenchmarkLastIndexHard3(b *testing.B) { benchmarkLastIndexHard(b, []byte("<b>hello world</b>")) }
+
 func BenchmarkSplitEmptySeparator(b *testing.B) {
 	for i := 0; i < b.N; i++ {
 		Split(benchInputHard, nil)