- change InstanceofExpression.resolveType() to register the
  instanceof type in the left hand expression if it is a
  CastExpression
- modify CastExpression to disable unnecessary type checking if
  an instanceof type has been registered and it is provably
  distinct from the expression type
- add new tests to CastTest

Change-Id: Ifb7730706b189915ef8afd859722ea831dccca76
Also-by: Stephan Herrmann <stephan.herrmann@berlin.de>