[js-perf-test] Move SubRegExp class definition outside loop

Defining the subclass within the loop significantly affects subsequent
test results. For instance, the Search benchmark is 50% slower if the
subclass is defined within the loop.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2537253003
Cr-Commit-Position: refs/heads/master@{#41384}

test/js-perf-test/RegExp/base_ctor.js

@@ -26,11 +26,12 @@ function CtorWithRegExpPatternAndFlags() {
   new RegExp(/[Cz]/, "guiym");
 }
 
-function CtorWithRegExpSubclassPattern() {
-  class SubRegExp extends RegExp {
+class SubRegExp extends RegExp {
   get source() { return "[Cz]"; }
   get flags() { return "guiym"; }
-  }
+}
+
+function CtorWithRegExpSubclassPattern() {
   new RegExp(new SubRegExp(/[Cz]/));
 }