[llvm] [NVPTX] Add intrinsics for new narrow FP conversions (PR #173954)

[Alex MacLean via llvm-commits]

================
@@ -659,6 +659,11 @@ let hasSideEffects = false in {
                 (ins B32:$src), (ins CvtMode:$mode),
                 "cvt${mode:base}.satfinite${mode:relu}." # F8Name # "x2.f16x2">,
       Requires<[hasPTX<81>, hasSM<89>]>;
+    def _bf16x2
+        : NVPTXInst<(outs B16:$dst), (ins B32:$src, CvtMode:$mode),
----------------
AlexMaclean wrote:

Nit: for most of these you can use `BasicFlagsNVPTXInst` (see the def directly above this one for an example). 

https://github.com/llvm/llvm-project/pull/173954