[GIsel][CombinerHelper] Don't consider debug insts in dominance queries [3/14]

Summary:
This fixes several issues where the presence of debug instructions could
disable certain combines, due to dominance queries finding uses/defs that
don't actually exist.

Reviewers: dsanders, fhahn, paquette, aemerson

Subscribers: hiraditya, arphaman, aprantl, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78253