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

[Phoebe Wang via llvm-commits]

================
@@ -3248,12 +3249,18 @@ bool SelectionDAGLegalize::ExpandNode(SDNode *Node) {
       Results.push_back(Tmp1);
     break;
   }
+  case ISD::STRICT_BF16_TO_FP:
----------------
phoebewang wrote:

They are different. The comment says the expansion is not allowed when strict mode is enforced. Here you just do the expansion unconditionally.

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