Fix dependency calculation for context and scope properties

We were incorrectly calculating writing to a context or scope property as a
dependency for an expression. We don't know whether a property is being written
only or also being read from at lookup time, but we can make that decision in
the isel then when generating the move instructions.

So initially context and scope properties end up in a candidate set first
and get promoted to real dependencies when they're being used in reading
moves.

Task-number: QTBUG-35210
Change-Id: Ia67057abafc2d611e1e6605327b4965ebe91cbed
Reviewed-by: Lars Knoll <lars.knoll@digia.com>