Special cases addition expressions where one of the sides is known to be a
string to enable chains of string additions to be transformed into a series
of ToPrimitiveToString operations followed by a single string concatenation 
at the end of the chain of additions. This should avoid creating temporary
strings for each of the string additions (in essence this is an automated
string builder).

BUG=v8:6243

Change-Id: I44977d6dad00ee906f251c4bd9cab27e160c09d1
Reviewed-on: https://chromium-review.googlesource.com/493966
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45453}