[Assignment Tracking][22/*] Add loop-deletion test

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

This test covers the NFC-for-normal-debug-info change D133303.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133319