Avoid using the iterator for arrays with fast elements where the iterator has
not been modified.

Only deals with the case where there is a single spread argument.

Improves the six-speed "spread" benchmark to 1.5x slower than baseline es5 implementation, compared to 19x slower previously.

BUG=v8:5511

Review-Url: https://codereview.chromium.org/2465253011
Cr-Commit-Position: refs/heads/master@{#40998}