This reverts commit 941d5f96.

Reason for revert: Regressed performance tests https://bugs.chromium.org/p/chromium/issues/detail?id=864540

Original change's description:
> [turbofan] Remove optimization for Cons strings
> 
> We used to have an optimized version for nodes that are concatenating
> two strings which was allocating an object on the heap, therefore
> preventing this code from being executed on the compiler thread.
> Octane benchmark results show insignificant increase in performance
> (< 0.5%) without this optimization - see
> https://docs.google.com/spreadsheets/d/1MC5NrMoMSsqxZqw0ojoZvomBb7q2EOt1S0sFoJ8ld2c/edit?usp=sharing
> which leads to the conclusion we can safely remove the optimization for now.
> 
> Bug: v8:7790
> Change-Id: I6492c6a76118cac568d28805995d55c5360bb123
> Reviewed-on: https://chromium-review.googlesource.com/1138246
> Reviewed-by: Georg Neis <neis@chromium.org>
> Commit-Queue: Maya Lekova <mslekova@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#54467}

TBR=jarin@chromium.org,neis@chromium.org,mslekova@chromium.org

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

Bug: v8:7790
Change-Id: I20a8a11e40bcd2bcfaf58154a1ab5e4daa7a25e4
Reviewed-on: https://chromium-review.googlesource.com/1143144
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54556}