Add an IC for CompareNil operations

Add an IC for CompareNil operations

This includes:
- Support for a "is_compiled" Code flag to distinguish Crankshafted code objects from their non-Crankshafted counterparts. This allows all variants of ICs and STUBs to either be Crankshafted or not without creating special Code Kinds or other code boilerplate gymnastics. 
- Modifying the IfBuilder to support continuations that can be used from generated ICs and from Hydrogen
- Changes to the IfBuilder to support "Or" and "And" expressions.
- Adding an "uninitialized" variant of Hydrogen code stubs that quickly call to the runtime miss code via a "LightWeightMiss" stub the first time rather than de-opting (for efficiency reasons).
- Remove all IsNilAndBranch platform-specific instructions and replace them with the corresponding sequence of smaller hydrogen "atoms".

[danno, 2013-04-10 12:23:40]

PTAL