[Assignment Tracking][23/*] Account for assignment tracking in SLP Vectorizer

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

The SLP-Vectorizer can merge a set of scalar stores into a single vectorized
store. Merge DIAssignID intrinsics from the scalar stores onto the new
vectorized store.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133320