There is no fast path for 0-length substrings, which implied runtime
calls for some of the trickier types of strings (such as non-flat
cons-strings). This made for a big performance gap between
String.p.slice and String.p.substring for those inputs.

This CL just adds the 0-length shortcut in CSA implementation
of SubString.

Here is an example where it makes difference:

  let s = "abcdefghijkalmnopqrst";
  let a = "";

  console.time("Substring");
  for (let i = 0; i < 100000; i++) {
    s += "0";
    a += s.substring(0, 0);
  }
  console.timeEnd("Substring");

Before this change:
> console.timeEnd: Substring, 640.308000

After this change:
> console.timeEnd: Substring, 13.242000

For completeness, here is the time for slice:
> console.timeEnd: Slice, 13.142000

This also recovers performance in the jsdom library, as
observed in https://github.com/jsdom/jsdom/issues/2350.

Bug: v8:6730
Change-Id: I7d2127e4506a86ec2a7ed378293476c3699dc47a
Reviewed-on: https://chromium-review.googlesource.com/c/1365282
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#58071}