[Assignment Tracking][17/*] Account for assignment tracking in memcpyopt

The Assignment Tracking debug-info feature is outlined in this RFC:

https://discourse.llvm.org/t/
rfc-assignment-tracking-a-better-way-of-specifying-variable-locations-in-ir

Maintain and propagate DIAssignID attachments in memcpyopt.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133312