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

[Nuno Lopes via llvm-commits]

nunoplopes wrote:

FWIW, I tried the suggested semantics on LLVM's test suite with Alive2 and it works fine. But without the fix also worked, so maybe there isn't sufficient coverage for these corner cases.

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