Use slightly smarter entry states for DV-init entry points

Summary: bsimmers noticed that we don't assert that the locals being
initialized in a DV-init are Uninit.  Easy enough.

Reviewed By: @bertmaher

Differential Revision: D1376676
Check if there's possibly a magic getter in thisPropAsCell

Reviewed By: @bertmaher

Differential Revision: D1375422