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

[via llvm-commits]

cmtice wrote:

I tested this PR on the case reported by blaikiew, where we had been seeing  a large regression in the size of .debug_lines.  With this PR we are no longer seeing the size regression.

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