[Assignment Tracking] Fix replaceVariableLocationOp for dbg.assign with DIArgList

[Assignment Tracking] Fix replaceVariableLocationOp for dbg.assign with DIArgList

The last time this function was updated DIArgLists were not supported for
dbg.assigns. Without this patch it's possible to now dereference an invalid
(the end) iterator. This occurs when the Address component gets replaced and
there's a DIArgList for the Value component (the contained values are
irrelevant).

The added unittest crashes without the code change in this patch.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D147922