Crankshaft just unconditionally deoptimizes the code when the length of
a string addition result would overflow. In order to protect against
deopt loops we insert a global protector cell.

We will use the same mechanism for inlining certain string additions
into TurboFan as well, and protecting against overflow (we will also
extend this to deal with String.prototype.concat and friends once we
get there).

BUG=v8:5404
R=jarin@chromium.org,hpayer@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.v8:v8_linux64_msan_rel

Committed: https://crrev.com/cb19257a926a55209a6d6858ce26d51a0447ba71
Review-Url: https://codereview.chromium.org/2348293002
Cr-Original-Commit-Position: refs/heads/master@{#39511}
Cr-Commit-Position: refs/heads/master@{#39525}