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';
+constcommon=require('../common.js');
+constv8=require('v8');
+
+constbench=common.createBenchmark(main,{
+size: [16,512,1024,4096,16386],
+millions: [1]
+});
+
+functionmain(conf){
+constiter=(conf.millions>>>0)*1e6;
+constsize=(conf.size>>>0);
+constb0=newBuffer(size).fill('a');
+constb1=newBuffer(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(vari=0;i<iter;i++){
+b0.compare(b1);
+}
+bench.end(iter/1e6);
+}