[es6] correct constant_elements backing-store when spread is used

[es6] correct constant_elements backing-store when spread is used

In order to ensure that the constant elements have the same length
as the initial constructed array, the constant elements backing store
length occasionally needs to be trimmed when spread elements are used.

BUG=v8:4298, 509889
LOG=N
R=adamk@chromium.org, rossberg@chromium.org

[caitp (gmail), 2015-07-14 02:57:28]

I'm not sure if this is the "right" fix for this problem, but it seems to work
locally, and corresponds with my findings of the cause of the bug.

[adamk, 2015-07-14 05:32:51]

I have a cleaner version of this in https://codereview.chromium.org/1225223004,
but it fails other spread tests.

[adamk, 2015-07-14 05:33:29]

On 2015/07/14 05:32:51, adamk wrote:
> I have a cleaner version of this in
https://codereview.chromium.org/1225223004,
> but it fails other spread tests.

By "cleaner" I just mean that it actually allocates a FixedArray of the correct
length initially instead of truncating the backing store after the fact.