[mir-strip-debug] Optionally preserve debug info that wasn't from debugify/mir-debugify

Summary:
A few tests start out with debug info and expect it to reach
the output. For these tests we shouldn't strip the debug info

Reviewers: aprantl, vsk, bogner

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77886