NFC. Move isDereferenceable to Loads.h/cpp

This is a part of the refactoring to unify isSafeToLoadUnconditionally and isDereferenceablePointer functions. In subsequent change I'm going to eliminate isDerferenceableAndAlignedPointer from Loads API, leaving isSafeToLoadSpecualtively the only function to check is load instruction can be speculated.   

Reviewed By: hfinkel

Differential Revision: http://reviews.llvm.org/D16180

llvm-svn: 261736