[llvm] [AMDGPU] Use nan as the identity for atomicrmw fmax/fmin (PR #97411)

[Matt Arsenault via llvm-commits]

arsenm wrote:

>  the same operation as llvm.maxnum/minnum

The definition for that is in flux at the moment. If it follows the busted ieee 754-2008 signaling nan behavior, there is no identity value. If the in-memory value is a signaling nan, the in-memory value will change to a quiet nan. 

Also, gfx12 changed the behavior of the instructions to the less broken 2019 minimumNumber. I think we need to fix the definition of atomicrmw fmin/fmax along with minnum/maxnum, and possibly add more opcodes corresponding to minimumnum/maximumnum from #96649

https://github.com/llvm/llvm-project/pull/97411