[llvm] [IndVars] Preserve flags of narrow IV inc if replacing with wider inc. (PR #80446)

[Florian Hahn via llvm-commits]

fhahn wrote:

@nikic WDYT of the updated checks? Not super happy with how they look, any ideas/suggestions how to improve them? Once we converged, I'll something similar as follow-up to https://github.com/llvm/llvm-project/pull/79512.

https://github.com/llvm/llvm-project/pull/80446