Check whether a pointer is non-null (isKnownNonNull) in isKnownNonZero.

This handles the case where we have an inbounds GEP with alloca as the pointer.
This fixes the regression in PR12750 and rdar://13286434.
Note that we can also fix this by handling some GEP cases in isKnownNonNull.

llvm-svn: 177321