[llvm] specify NaN behavior more precisely (PR #66579)

[Joshua Cranmer via llvm-commits]

jcranmer-intel wrote:

> I've a question regarding fpext & fptrunc: what's the payload we want there? We can't just copy the inputs.

`fpext` and `fptrunc` correspond to the IEEE 754 operation **convertFormat**, so the rules related to SNaN handling need to apply (i.e., it may or may not be quieting input SNaN). IEEE 754 furthermore recommends NaN propagation as follows:

> Conversion of a quiet NaN from a narrower format to a wider format in the same radix, and then back to
the same narrower format, should not change the quiet NaN payload in any way except to make it
canonical.

The natural way to implement this is to designate certain bits to chop, but I don't know what the full suite of implementations here is. In x86, the afflicted bits are the LSB, so `fpext` on `0xffffffff` results in `0xffffffffe0000000`, and I think `APFloat` is following the same rules here, but I'm not certain all NaN-propagating implementations would follow the same rules.

https://github.com/llvm/llvm-project/pull/66579