[llvm] [SelectionDAG] Avoid double rounding by using a libcall for int (>i32) to bf16 conversions (PR #71658)

[Eli Friedman via llvm-commits]

efriedma-quic wrote:

I thought the previous discussion concluded you need special handling for integers larger than 24 or so bits?  (I suggested we might be able to special-case 32-bit integers using double-precision ops, but we'd need to explicitly code that, I think; the default handling won't work.)

https://github.com/llvm/llvm-project/pull/71658