Adds typed lowering of JSStringConcat to ConsString allocation if the
following conditions hold:
 - All concatinations will result in a ConsString of >= ConString::kMinLength
 - No concatinations will result in a empty string in the RHS unless there is
   a sequential string in the LHS.

This also means JSStringConcat needs an eager checkpoint since it can
deopt if throwing a RangeError when the string length protector is valid.

BUG=v8:6243

Change-Id: I01ca79f884df467c10f2c032c72d51b5199c1a3c
Reviewed-on: https://chromium-review.googlesource.com/526636
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46093}