[llvm] [InstCombine] Fold more 'fcmp' 'select' instrs idioms into 'fabs' (PR #83381)

[Yashwant Singh via llvm-commits]

yashssh wrote:

> I'm wondering whether we should be propagating the function-level attributes to instruction flags _somewhere_ so that this is handled generically, rather than in individual folds?

That can be done but will be a separate work altogether, my understanding is somewhere early in the pipeline. I was also suggested that it would require allowing `loads` and `stores` to have `fast` flags so that they are safely reflected in `phi`s and `select`s. 

https://github.com/llvm/llvm-project/pull/83381