This is a reland of e70cbb83

Moved the ConsString comparison logic out-of-line, both to make gcc
happy, and to reduce the size of the fast-path in IsEqualToImpl.

Original change's description:
> [string] Fix non-SeqStrings in IsEqualTo
>
> Bug: chromium:1193903
> Change-Id: I80704dd3cba5754779432356b20bd3ea99630291
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2794426
> Commit-Queue: Leszek Swirski <leszeks@chromium.org>
> Commit-Queue: Igor Sheludko <ishell@chromium.org>
> Reviewed-by: Igor Sheludko <ishell@chromium.org>
> Auto-Submit: Leszek Swirski <leszeks@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#73746}

Bug: chromium:1193903
Change-Id: Iae6f078853438427e86d3ac68bcfed0712a85bf7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2797288
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73763}