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

[Phoebe Wang via llvm-commits]

phoebewang wrote:

> @phoebewang Does X86 have lib calls for `STRICT_BF16_TO_FP` and `STRICT_FP_TO_BF16`? I'm trying to find a target to test the lowering via the lib call path.

Do you mean this? https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/RuntimeLibcalls.def#L322

I see you use the same library as none strict one. But we don't provide libcall for extend.

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