The JSArgumentsObject::length representation is initially Smi, so we can
record that on the initial map and use it to optimize the accesses in
TurboFan based on that. Similar for JSSloppyArgumentsObject::caller.

BUG=v8:6262
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2810333004
Cr-Commit-Position: refs/heads/master@{#44644}