[llvm] [DAG] computeKnownFPClass - add ISD::SINT_TO_FP/UINT_TO_FP handling (PR #190539)

[Pau Sum via llvm-commits]

pau-sum wrote:

> You're on the right track, but I'd expect it to handle as many similar cases to valuetracking's implementation as possible - in particular its never negzero/subnormal and never infinity if the integer src bitwidth is small enough.

I copied the implementation from [ValueTracking](https://github.com/llvm/llvm-project/blob/58208a0cc165c23d2fad4ed46950b342156875ac/llvm/lib/Analysis/ValueTracking.cpp#L5770-L5826). I am not too confident if I wrote good test cases. Let me know what you think.

https://github.com/llvm/llvm-project/pull/190539