When Crankshaft compiles a keyed load to arguments, it disabled
optimization unless the KEYED_LOAD_IC for the access was monomorphic.
But that's too restrictive, since it will also disable optimization
for this function when the access is on a path that was never executed
so far.

This was spotted in the Node.js core function EventEmitter.prototype.emit,
which was no longer optimizable with Crankshaft using latest V8.

R=jarin@chromium.org
BUG=v8:5790

Review-Url: https://codereview.chromium.org/2607303002
Cr-Commit-Position: refs/heads/master@{#42005}