This is a reland of e7e512da

Original change's description:
> [turbofan] Add fast path for single-character String#startsWith()
> 
> This CL adds a fast path to String#startsWith(s) if s is a
> single character string.
> 
> Bug: v8:8400
> Change-Id: Ibd6a9d1e46d98f41c198d2b579208e25003eedb0
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1525362
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#61504}

Bug: v8:8400
Change-Id: Ic2d60ccb8fdeb51373fcd025a7e970fda0c14d79
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1618342Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61637}