Previously all internal accesses to the String::length field in TurboFan
would use the StringLength operator, whereas explicit `string.length`
accesses from user JavaScript code would use LoadField operators instead.
This inconsistency led to redundant loads of the String::length, for
example in case of code like

```js
subject.substring(1, subject.length - 1)
```

where the `subject.substring` call introduces a StringLength(subject)
node, and the `subject.length` introduces a LoadField[length](subject)
node.

Consistently using StringLength operator everywhere enables
optimizations in TurboFan that had been blocked before here (besides
avoiding the redundant load operations).

Bug: v8:8015
Change-Id: I21c82bc418105b9933a9e60dde11c7b222dfcf4f
Reviewed-on: https://chromium-review.googlesource.com/1212942
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55710}