This reverts commit 93d67d20.

Reason for revert: several layout test failures:
https://build.chromium.org/p/client.v8.fyi/builders/V8-Blink%20Linux%2064/builds/21062

Original change's description:
> [turbofan] Inline StringCharCodeAt like Crankshaft did.
> 
> 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/873376
> Reviewed-by: Yang Guo <yangguo@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#50702}

TBR=yangguo@chromium.org,bmeurer@chromium.org

Change-Id: I6e909adba82adc46e269ab14426ee24caaca6ff9
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: v8:7326
Reviewed-on: https://chromium-review.googlesource.com/875963Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50709}