[Assignment Tracking][19/*] Account for assignment tracking in ADCE

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

In an attempt to preserve more info, don't delete dbg.assign intrinsics that
are considered "out of scope" if they're linked to instructions.

Reviewed By: jmorse

Differential Revision: https://reviews.llvm.org/D133314