[lworld] RFR(M): Cleanup of null checks

[Tobias Hartmann]

Hi,

please review the following patch:
http://cr.openjdk.java.net/~thartmann/valhalla/lworld/null_check_cleanup/

Changes:
- Moved all null checks into ValueTypeNode::make_from_oop(). We are now emitting null checks if the
oop can be null
- Added asserts at the call sites to verify that we never emit null checks where it's not required
- Fixed a problem with OSR where a null value type is accessed without a null check (see new test in
TestOnStackReplacement.java)
- Some refactoring and comment cleanups

All compiler and runtime tests pass.

Thanks,
Tobias