[Assignment Tracking] Ignore zero-sized fragments

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