[llvm] [DebugInfo] Don't apply is_stmt on MBB branches that preserve lines (PR #108251)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM, I still worry about the clarity of the opening comment (just because there's a lot going on), is there an easy example that can be cooked up to demonstrate this? Fine to land without.

https://github.com/llvm/llvm-project/pull/108251