https://crrev.com/c/3571817 introduced a bug that string table lookups
failed on SlicedStrings with a start offset of 0.
This CL fixes the issue by re-using the already computed hash only
if the length of the source string matches the length of the string to
lookup.

Bug: chromium:1320179, chromium:1321573
Change-Id: Ic8755a0266a9ec67fe5eb9c96fdab1b55d5009f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616723
Auto-Submit: Patrick Thier <pthier@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80309}