During JSTypedLowering, when we see a JSAdd where we know that at least
one side is already a String, we can try to strength-reduce the other
side to a string as well. And once we have that, check whether both
sides are now String constants, and if the concatenation won't overflow
the string length limit, we can just constant-fold the StringAdd.

This improves the Six Speed template_string benchmarks by up to 5x, as
we no longer need to perform the String concatenations on every loop
iteration.

Bug: v8:6815
Change-Id: I8c47b2adf66b585d2f191cf805604b435f6256cd
Also-By: jarin@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/663181
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47974}