ObjCARC: Don't look at users of ConstantData

Stop looking at users of UndefValue and ConstantPointerNull in the
objective C ARC optimizers.  The other users aren't actually
interesting, since they're not pointing at a particular object.  I
imagine these calls could be optimized through -instcombine... maybe
they already are?

These early returns will be required at some point in the future, with a
WIP patch that asserts when someone accesses a use-list on ConstantData.

llvm-svn: 282338