[js-perf-test] Add RegExp microbenchmarks

test/js-perf-test/RegExp/base_test.js

+// Copyright 2016 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+load("base.js");
+
+var str;
+var re;
+
+function SimpleTest() {
+  re.test(str);
+}
+
+function Test1Setup() {
+  re = /[Cz]/;
+  str = createHaystack();
+}
+
+function Test2Setup() {
+  re = /[cZ]/;  // Not found.
+  str = createHaystack();
+}
+
+var benchmarks = [ [SimpleTest, Test1Setup],
+                   [SimpleTest, Test2Setup],
+                 ];