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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

So before landing this, I'll note the state of its [performance characteristics](http://llvm-compile-time-tracker.com/compare.php?from=bc7e5c2016e287b768d2a3a1de15f6bb644622ae&to=52d703cd46e5b1ece3dc524b501e5e1cb9412f32&stat=instructions%3Au) in case anyone comes looking after it's landed - for optimized debug builds, the performance cost of this patch is negligible. For O0 debug builds, it has a performance cost of around 0.15%; I don't see too many ways of optimizing it further without compromising correctness in some cases though, since AFAICT just the cost of iterating over the instructions is enough to noticeably slow down O0 builds. Since this is necessary to fix an O0 debug info bug (https://github.com/llvm/llvm-project/issues/104695) I think the cost is probably worth it.

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