[MachineBasicBlock] Add helpers for skipping debug instructions [1/14]

Summary:
These helpers are exercised by follow-up commits in this patch series,
which is all about removing CodeGen differences with vs. without debug
info in the AArch64 backend.

Reviewers: fhahn, aprantl, jpaquette, paquette

Subscribers: kristof.beyls, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78260