[globalisel] Allow backends to report an issue without triggering fallback. NFC

Summary:
This will allow us to fix the issue where the lost locations
verifier causes CodeGen changes on lost locations because it
falls back on DAGISel

Reviewers: qcolombet, bogner, aprantl, vsk, paquette

Subscribers: rovka, hiraditya, volkan, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78261