Such dbg.assigns will occur if you write zero-sized memcpys (see
https://reviews.llvm.org/D146987#4240016).
Handle this in AssignmentTrackingAnalysis (back end) rather than
AssignmentTrackingPass (declare-to-assign) in case it is possible to reproduce
this as a result of optimisations.
Reviewed By: jmorse
Differential Revision: https://reviews.llvm.org/D147435