DirectArguments::create needs to initialize to undefined instead of the empty value

DirectArguments::create needs to initialize to undefined instead of the empty value
https://bugs.webkit.org/show_bug.cgi?id=186818
<rdar://problem/38415177>

Reviewed by Filip Pizlo.

JSTests:

* stress/create-direct-arguments-in-osr-should-initialize-to-undefined.js: Added.
(foo):
(bar):

Source/JavaScriptCore:

The bug here is that we will emit code that just loads from DirectArguments as
long as the index is within the known capacity of the arguments object (op_get_from_arguments).
The arguments object has at least enough capacity to hold the declared parameters.
When we materialized this object in OSR exit, we initialized up to to the capacity
with JSValue(). In OSR exit, though, we only filled up to the length of the
object with actual values. So we'd end up with a DirectArguments object with
capacity minus length slots of JSValue(). To fix this, we need initialize up to
capacity with jsUndefined during construction. The invariant of this object is
that the capacity minus length slots at the end are filled in with jsUndefined.

* runtime/DirectArguments.cpp:
(JSC::DirectArguments::create):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@233000 268f45cc-cd09-0410-ab3c-d52691b4dbfc