This replaces the previous CheckStringAdd operator which deopts in case
the combined length overflows with a dedicated pure StringConcat operator.
This operator is similar to NewConsString in that it takes the resulting
length plus the two input strings. The operator relies on the length
being checked explicitly by the surrounding code instead of baking the
check into the operator itself. This way TurboFan can eliminate
redundant/unnecessary StringConcat operations, since they are pure now.

This also unifies the treatment of string addition in JSTypedLowering,
and generalizes the StringLength constant-folding to apply to more cases
not just the JSAdd cases inside JSTypedLowering.

Bug: v8:7902, v8:8015
Change-Id: I987ec39815a9464fd5fd9c4f7b26b709f94f2b3f
Reviewed-on: https://chromium-review.googlesource.com/1213205Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55725}