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

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> Is there an argument that it might be reasonable to leave this decision to the debugger?

Possibly, but there might be a justification to do this regardless for the purposes of reducing file size. This patch has a performance cost of 0.17% for `-O0 -g` builds (no notable increase for ReleaseLTO-g) with a self-built clang, which suggests some further optimization is required (likely narrowing down the cases that we'll be willing to handle). However, the original patch gave a 0.33% size increase at `-O0 -g`, and a 0.17% at ReleaseLTO-g, which this patch brings down to 0.01% and 0.04% respectively. I haven't managed to reproduce the change in file size noted by @dwblaikie on the previous PR https://github.com/llvm/llvm-project/pull/105524, but if the effect of this patch is proportional then even the performance cost of this patch could be considered worthwhile.

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