The hand-written KeyedLoadIC_Megamorphic stub didn't care about JSArray
lengths, which made it lenient towards said lengths being wrong, but it
will soon fix that bug and thereby become more strict.

LiveEdit: factory->NewJSArray(capacity) doesn't set a length, so set it
manually.
RegExp: to avoid having to take care of array length updating in the
RegExpExecStub, just use a JSObject instead.

Review-Url: https://codereview.chromium.org/2244673002
Cr-Commit-Position: refs/heads/master@{#38624}