Adds a CheckString to all operand inputs of JSStringConcat. The operands are
already known to be strings, so this will get eliminated in almost all cases,
however, if there is a yield within the concatenation then we lose the
knowledge that the previous operands are strings since the values are loaded
from the generator object. Adds a test for this case.

BUG=v8:6243

Change-Id: I1601a316e6efbed1c53486f1027cb0ea023ff030
Reviewed-on: https://chromium-review.googlesource.com/549301
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46243}