[llvm] clarify NaN propagation in fptrunc (PR #68554)

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

So, discarding all 1-bits in the payload would give an Inf rather than a NaN then.

FWIW, I tried x86, and it returns the canonical quiet NaN: https://gcc.godbolt.org/z/o1K6rrov9
Same for an ARM server I've access to.

https://github.com/llvm/llvm-project/pull/68554