This creates a uniform PerformPromiseThen builtin, which performs the
operation with the same name from the spec, except that it expects the
handlers to be either undefined or callable already, since this is only
relevant for a single callsite (namely Promise.prototype.then).

Introduce a matching operator JSPerformPromiseThen into TurboFan, which
represents this operation and removes the additional checks in case of
Promise.prototype.then based on the information we can derived from the
receiver maps.

This yields a nice 20-25% improvement on Promise.prototype.then, as
illustrated by the following micro-benchmark

```js
const N = 1e7;
function inc(x) { return x + 1; }
function chain(promise) {
  return promise.then(inc).then(value => {
      if (value < N) chain(Promise.resolve(value));
    });
}
console.time('total');
chain(Promise.resolve(0));
setTimeout(console.timeEnd.bind(console, 'total'));
```

which goes from around 1230ms to 930ms with this patch.

Bug: v8:7253
Change-Id: I5712a863acdbe7da3bb8e621887c7b952148c51a
Reviewed-on: https://chromium-review.googlesource.com/899064Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51071}