slices: add benchmark for IsSorted vs. IntsAreSorted

We'd like to mention in a comment that users should prefer
slices.IsSorted over sort.IntsAreSorted and similar
functions. Create a benchmark that shows this.

goos: linux
goarch: amd64
pkg: slices
cpu: 11th Gen Intel(R) Core(TM) i5-1135G7 @ 2.40GHz
BenchmarkIntsAreSorted-8             	    6031	    198315 ns/op
BenchmarkIsSorted-8                  	   26580	     45801 ns/op

Change-Id: I4f14fafd799ecec35c8a5215b74994e972103061
Reviewed-on: https://go-review.googlesource.com/c/go/+/502556
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Eli Bendersky‎ <[email protected]>
Reviewed-by: Eli Bendersky <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: eliben

src/slices/sort_benchmark_test.go

@@ -77,6 +77,24 @@ func BenchmarkSlicesSortInts_Reversed(b *testing.B) {
 	}
 }
 
+func BenchmarkIntsAreSorted(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		ints := makeSortedInts(N)
+		b.StartTimer()
+		sort.IntsAreSorted(ints)
+	}
+}
+
+func BenchmarkIsSorted(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		b.StopTimer()
+		ints := makeSortedInts(N)
+		b.StartTimer()
+		IsSorted(ints)
+	}
+}
+
 // Since we're benchmarking these sorts against each other, make sure that they
 // generate similar results.
 func TestIntSorts(t *testing.T) {