[llvm] WIP DO NOT MERGE Fix missing debug info with tail merging (PR #94715)

[Alan Zhao via llvm-commits]

alanzhao1 wrote:

This breaks `llvm/test/DebugInfo/COFF/local-variables.ll`, which has to do with COFF debug info.

I'm still trying to figure out what exactly is going on here - it seems like the existing test is missing debug info, but there's also some weirdness with basic blocks being added...

https://github.com/llvm/llvm-project/pull/94715