This is a change from ES5 to ES6: When reversing an array, first it is checked
whether the element exists, before the element is looked up. The order in ES6
is

[[HasElement]] lower
[[Get]] lower (if present)
[[HasElement]] upper
[[Get]] upper (if present)

In ES5, on the other hand, the order was

[[Get]] lower
[[Get]] upper
[[HasElement]] lower
[[HasElement]] upper

To mitigate the performance impact, this patch implements a new, third copy
of reversing arrays if %_HasPackedElements. This allows us to skip all
membership tests, and a quick and dirty benchmark shows that the new version
is faster:

Over 4 runs, the slowest for the new version:
d8> var start = Date.now(); for (var i = 0; i < 100000000; i++) [1, 2, 3, 4, 5].reverse(); Date.now() - start
4658

Over 3 runs, the fastest for the old version:
d8> var start = Date.now(); for (var i = 0; i < 100000000; i++) [1, 2, 3, 4, 5].reverse(); Date.now() - start
5176

BUG=v8:4223
R=adamk
LOG=Y

Review URL: https://codereview.chromium.org/1238593003

Cr-Commit-Position: refs/heads/master@{#29716}