This avoids the call to the StringCharCodeAt builtin from
within TurboFan optimized code and instead emits a loop
that does the character load. This (together with previously
reverted CL to the JSCallReducer) almost completely recovers
the performance regression caused when we shipped TurboFan.

Without untrusted code mitigations the benchmark goes from
580ms to roughly 490ms, and with the patch to the JSCallReducer
the time goes down to 280ms, which is very close to what we
had with Crankshaft.

This also renames the LoadFromString helper method in the
EffectControlLinearizer to LoadFromSeqString to make it
clear what it does.

Bug: v8:7326
Change-Id: Ibe0ec1847911a234f244bd8dcec6be18b241fda0
Reviewed-on: https://chromium-review.googlesource.com/873376Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50702}