[llvm] [IR] LangRef: state explicitly that floats generally behave according to IEEE-754 (PR #102140)

[Ralf Jung via llvm-commits]

RalfJung wrote:

@nikic @arsenm maybe you could take a look? I am not sure who'd be a good reviewer for this. :)

https://github.com/llvm/llvm-project/pull/102140