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/1138246Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54467}