[llvm] Attributor: Add denormal-fp-math to attributor-light (PR #79576)

[Matt Arsenault via llvm-commits]

arsenm wrote:

> Can you please provide more background for this change? I thought AttributorLight was supposed to mirror FunctionAttrs in terms of supported attributes.

The main point is I need this done in the default pass pipeline. I'm doing it here as new-FunctionAttrs, as theoretically FunctionAttrs will be obsoleted eventually and I don't want to waste effort implementing it twice 

https://github.com/llvm/llvm-project/pull/79576