Previously AccessorAssembler::HandlePolymorphicCase() had 4 versions of
the inner loop unrolled, but we always had to check against the length
after 1 (POLYMORPHIC with name) or 2 (regular POLYMORPHIC) unrolled
iterations anyways, so there's not a lot of benefit to unrolling besides
the potentially better branch prediction in some cases. But that doesn't
seem to be beneficial even in extreme cases (in fact on ARM cores we
might get some benefit from having less code instead), and probably
doesn't justify the additional C++ / generated code.

I used the following extreme micro-benchmark to check the worst case
performance impact:

```js
function test(o, n) {
  var result;
  for (var i = 0; i < n; ++i) {
    result = o.x;
  }
  return result;
}

const N = 1e8;
const objs = [{x: 0}, {x:1,a:1}, {x:2,b:2}, {x:3,c:3}];
for (var j = 0; j < objs.length; ++j) test(objs[j], N);

console.time('Time');
for (var j = 0; j < objs.length; ++j) test(objs[j], N);
console.timeEnd('Time');
```

Running this with --noopt shows a ~1% performance regression with this
patch on a beefy z840 gLinux workstation, which gives me some confidence
that overall this patch is going to be neutral and maybe beneficial in
case of less powerful ARM cores.

Note to performance sheriffs: This could potentially tank some
performance tests. In that case we may need to revisit the unrolling.

Bug: v8:8562
Change-Id: I731599a7778da1992d981d36022c407ef5c735eb
Reviewed-on: https://chromium-review.googlesource.com/c/1448275Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59252}