This adds microbenchmarks for:

* the RegExp constructor,
* flag accessors,
* and RegExp functions: exec, @@match, @@search, test, @@split, @@replace.

Each benchmark is further split to measure fast and slow paths.

BUG=v8:5339

Review-Url: https://codereview.chromium.org/2521263003
Cr-Commit-Position: refs/heads/master@{#41285}