Note: This CL might regress code that relies on such arguments access.
In that case, we could still optimize the access if it accesses at
constant index (and the argument at that index is not context-allocated).

If any code relies on a general access to context-allocated arguments,
we would need to analyze the function for assignment to the arguments - this
might be quite tricky.

BUG=chromium:662845

Review-Url: https://codereview.chromium.org/2484723002
Cr-Commit-Position: refs/heads/master@{#40813}