[js-perf-test] Add regression benchmark

This CL adds a regression benchmark for a fast-path of
String.p.charCodeAt, which is important for node.js.

Bug: v8:7326
Change-Id: I54efaa2988c595dd40e6a55a3464b3ee7de6f07b
Reviewed-on: https://chromium-review.googlesource.com/942885
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51654}

test/js-perf-test/JSTests.json

@@ -221,7 +221,8 @@
             {"name": "StringEndsWith"},
             {"name": "StringIncludes"},
             {"name": "StringFromCodePoint"},
-            {"name": "StringCodePointAt"}
+            {"name": "StringCodePointAt"},
+            {"name": "StringCodePointAtSum"}
           ]
         },
         {

test/js-perf-test/Strings/harmony-string.js

@@ -32,6 +32,11 @@ new BenchmarkSuite('StringCodePointAt', [1000], [
     CodePointAt, CodePointAtSetup, CodePointAtTearDown),
 ]);
 
+new BenchmarkSuite('StringCodePointAtSum', [100000], [
+  new Benchmark('StringCodePointAtSum', false, true, 3,
+    CodePointAtSum, CodePointAtSumSetup),
+]);
+
 
 var result;
 
@@ -125,3 +130,16 @@ function CodePointAt() {
 function CodePointAtTearDown() {
   return result === (MAX_CODE_POINT / K) * ((MAX_CODE_POINT / K) + 1) / 2;
 }
+
+var payload;
+
+function CodePointAtSumSetup() {
+  payload = "abcdefghijklmnopqrstuvwxyz";
+  for(var j = 0; j < 16; ++j) payload += payload;
+}
+
+function CodePointAtSum() {
+  var c = 0;
+  for(j=payload.length-1; j >=0; --j) c+=payload.charCodeAt(j);
+  return c;
+}