math/bits: correct BenchmarkSub64

Previously, the benchmark was measuring Add64 instead of Sub64.

Change-Id: I0cf30935c8a4728bead9868834377aae0b34f008
Reviewed-on: https://go-review.googlesource.com/c/144380Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/math/bits/bits_test.go

@@ -918,7 +918,7 @@ func BenchmarkSub32(b *testing.B) {
 func BenchmarkSub64(b *testing.B) {
 	var z, c uint64
 	for i := 0; i < b.N; i++ {
-		z, c = Add64(uint64(Input), uint64(i), c)
+		z, c = Sub64(uint64(Input), uint64(i), c)
 	}
 	Output = int(z + c)
 }