benchmark: add benchmark for buf.compare()

There is a benchmark for the class method `Buffer.compare()` but not for
the instance method `buf.compare()`. This adds that benchmark.

I used this to confirm a performance regression in an implementation I
was considering. While the implementation was a bust, it does seem like
the benchmark is worthwhile.

The benchmark is nearly identical to the existing `Buffer.compare()`
benchmark except, of course, that it calls `buf.compare()` instead.

PR-URL: #5441
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>

Author: Trott

benchmark/buffers/buffer-compare-instance-method.js

@@ -0,0 +1,29 @@
+'use strict';
+const common = require('../common.js');
+const v8 = require('v8');
+
+const bench = common.createBenchmark(main, {
+  size: [16, 512, 1024, 4096, 16386],
+  millions: [1]
+});
+
+function main(conf) {
+  const iter = (conf.millions >>> 0) * 1e6;
+  const size = (conf.size >>> 0);
+  const b0 = new Buffer(size).fill('a');
+  const b1 = new Buffer(size).fill('a');
+
+  b1[size - 1] = 'b'.charCodeAt(0);
+
+  // Force optimization before starting the benchmark
+  b0.compare(b1);
+  v8.setFlagsFromString('--allow_natives_syntax');
+  eval('%OptimizeFunctionOnNextCall(b0.compare)');
+  b0.compare(b1);
+
+  bench.start();
+  for (var i = 0; i < iter; i++) {
+    b0.compare(b1);
+  }
+  bench.end(iter / 1e6);
+}