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}

This reverts commit e7e512da.

Reason for revert: chromium:963687, chromium:963731

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}

TBR=sigurds@chromium.org,bmeurer@chromium.org,usharma1998@gmail.com

Change-Id: Ie09a96557102063ab15c3900563cf16ec57a47e3
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:8400
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1614605Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61562}

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/+/1525362Reviewed-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}

This reverts commit acadb202.

Reason for revert: chromium:941952

Original change's description:
> [turbofan] add fast path for String.p.startsWith
> 
> Add a fast path for String.p.startsWith(str) when length of str is 1.
> 
> Bug: v8:8400
> Change-Id: I65e657549902dc3ad064a213d815dd098ce6455f
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1491872
> Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
> Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#60091}

TBR=sigurds@chromium.org,bmeurer@chromium.org,usharma1998@gmail.com

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: v8:8400
Change-Id: Iabd66b8db0d6958686ea15a042d8567955700c9e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524205
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60279}

Add a fast path for String.p.startsWith(str) when length of str is 1.

Bug: v8:8400
Change-Id: I65e657549902dc3ad064a213d815dd098ce6455f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1491872
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60091}