Make context-sensitive isDereferenceable queries in isSafeToLoadUnconditionally

This is a part of the refactoring to unify isSafeToLoadUnconditionally and isDereferenceablePointer functions. In the subsequent change isSafeToSpeculativelyExecute will be modified to use isSafeToLoadUnconditionally instead of isDereferenceableAndAlignedPointer.   

Reviewed By: reames

Differential Revision: http://reviews.llvm.org/D16227

llvm-svn: 260520