[Assignment Tracking][16/*] Account for assignment tracking in mldst-motion

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

mldst-motion will merge and sink the stores in if-diamond branches into the
common successor. Attach a merged DIAssignID to the merged store.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133311