This makes maybe_assigned correct (instead of being overly pessimistic
in the following case):

function f() { function g() { arguments; }; }

(Tests upcoming as part of https://codereview.chromium.org/2580833005 )

BUG=v8:5501, v8:5678
R=verwaest@chromium.org, neis@chromium.org

Review-Url: https://codereview.chromium.org/2579303002
Cr-Commit-Position: refs/heads/master@{#41787}