[llvm] CodeGen: Strengthen definition of F{MIN|MAX}NUM_IEEE nodes (PR #85195)

[Qiu Chaofan via llvm-commits]

ecnelises wrote:

The semantics change looks good to me, but I'm still feeling it's (naming?) a little weird: `FMAXNUM` follows 2008 rules, `FMAXIMUM` follows 2019 rules, while `FMAXNUM_IEEE` follows a mix of 2008 and 2019 to match current hardware behavior (and make lowering easier). Will some target be an exception of such assumption?

https://github.com/llvm/llvm-project/pull/85195