RFR: 8134797: Remove explicit casts in CollectorPolicy hierarchy

[Kim Barrett]

Please review this change to eliminate explicit casts within the
CollectorPolicy hierarchy.  All of the casts being eliminated were
C-style casts; there are no C++ style casts in this hierarchy.

For downcasts to GenCollectorPolicy* the policy was being obtained
from a GenCollectedHeap, so we just use gen_policy instead of an
unchecked downcast of collector_policy.

For the remaining downcast (to ConcurrentMarkSweepPolicy*) we now
assert the corresponding is_xxx_policy and use as_xxx_policy to
perform the conversion.

Changed the explicit upcasts to CollectorPolicy* to implict
conversions.  For the problematic one, moved to a scope where
G1CollectorPolicy is complete, so the now implicit conversion works.
I'm kind of surprised that one didn't cause obvious problems.

CR:
https://bugs.openjdk.java.net/browse/JDK-8134797

Webrev:
http://cr.openjdk.java.net/~kbarrett/8134797/webrev.00/

Testing:
jprt