Before this change, the spread desugaring would naively call
`%AppendElement($R, the_hole)` and in some cases $R would have
a non-holey elements kind, putting the array into the bad state
of exposing holes to author code.

This patch avoids calling %AppendElement with a hole, instead
simply incrementing $R.length when it sees a hole in the literal
(this is safe because $R is known to be an Array). The existing
logic for elements transitions takes care of giving the array a
holey ElementsKind.

BUG=chromium:644215

Review-Url: https://codereview.chromium.org/2321533003
Cr-Commit-Position: refs/heads/master@{#39294}