Adds constant folding for the StringConcat bytecode to
NativeContextSpecialization. Can reduce operator to either a fully folded
constant string, or a JSAdd or a StringConcat with a reduced number of
operators.

BUG=v8:6243, chromium:738312

Change-Id: I6b2be6a3d95230a23f3c7390a4f7be5181c49a2a
Reviewed-on: https://chromium-review.googlesource.com/559146
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46461}