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

[Andy Kaylor via llvm-commits]

andykaylor wrote:

> One annoying bit about handling this as a parameter attribute is we can't attach the same to other arbitrary sources (e.g. load) which should behave exactly the same. We would need another mirror metadata

Yes, not being able set the flag on loads is a problem. In cases where SROA is going to eliminate a store followed immediately by a load, but I don't see any reason the `nsz` property wouldn't also apply to any loads of floating-point data from other sources within the scope where the setting is active.

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