[llvm] [SelectionDAG] Add `STRICT_BF16_TO_FP` and `STRICT_FP_TO_BF16` (PR #80056)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

To make progress, can you split the non-lib call expansion to a separate change? It's easier to start with the boilerplate and case that doesn't require writing new infrastructure 

https://github.com/llvm/llvm-project/pull/80056