[Assignment Tracking][14/*] Account for assignment tracking in instcombine

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

Most of the updates here are just to ensure DIAssignID attachments are
maintained and propagated correctly.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133307