When rtt.sub is called repeatedly with the same arguments, it
should return the same result. This CL introduces a cache for
previously created sub-RTTs to achieve that.

Bug: v8:7748
Change-Id: Ie6c74eedf0df6f94cd973fdb0b6b6fc0130a9c41
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2275967
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#68715}