[compiler] Optimize String.prototype.startsWith

We could optimize String#startsWith in JSCallReducer for
three conditions:
- If search_element is definitely not a string, we make no change.
- If search_element is definitely a string and its length is less
or equal than max inline matching sequence threshold, we could
inline the entire matching sequence.
- Else we try to inline, and have a runtime deopt if search_element
is not a string.

Bug: v8:8400
Change-Id: I505090b91d35fbc2c91cdf985717c68135cba807
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3517936


Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79621}