[llvm] [StackColoring] Do not merge stack slots with `returns_twice` call-sites (PR #196542)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

> I don't think you need to disable stack coloring entirely, isn't it sufficient to disable the first-use heuristic? I.e. treat all stack slots as "conservative".

That makes more sense, updated, thanks!

https://github.com/llvm/llvm-project/pull/196542