AMDGPU: Make v2f32 -> v2f16 legal when target supports v_cvt_pk_f16_f32 (llvm#139956) (llvm#2478)

If targets support v_cvt_pk_f16_f32 instruction, v2f32 -> v2f16 should
be legal. However, SelectionDAG does not allow us to specify the source
type in the legalization rules. To workaround this, we make FP_ROUND
Custom for v2f16 then set up v2f32 -> v2f16 to be legal during custom
lowering.

Cherry-pick llvm#139956 to mainline
(after bulk promotion)

Fixes: SWDEV-532608 -- expected v_cvt_pk_f16_f32 was not generated.

Author: changpeng

llvm/lib/Target/AMDGPU/SIISelLowering.cpp

@@ -916,6 +916,9 @@ SITargetLowering::SITargetLowering(const TargetMachine &TM,
     setOperationAction(ISD::BUILD_VECTOR, MVT::v2bf16, Legal);
   }
 
+  if (Subtarget->hasCvtPkF16F32Inst())
+    setOperationAction(ISD::FP_ROUND, MVT::v2f16, Custom);
+
   setTargetDAGCombine({ISD::ADD,
                        ISD::UADDO_CARRY,
                        ISD::SUB,
@@ -6807,11 +6810,18 @@ SDValue SITargetLowering::getFPExtOrFPRound(SelectionDAG &DAG, SDValue Op,
 }
 
 SDValue SITargetLowering::lowerFP_ROUND(SDValue Op, SelectionDAG &DAG) const {
+  SDValue Src = Op.getOperand(0);
+  EVT SrcVT = Src.getValueType();
+  EVT DstVT = Op.getValueType();
+
+  if (DstVT == MVT::v2f16) {
+    assert(Subtarget->hasCvtPkF16F32Inst() && "support v_cvt_pk_f16_f32");
+    return SrcVT == MVT::v2f32 ? Op : SDValue();
+  }
+
   assert(Op.getValueType() == MVT::f16 &&
          "Do not know how to custom lower FP_ROUND for non-f16 type");
 
-  SDValue Src = Op.getOperand(0);
-  EVT SrcVT = Src.getValueType();
   if (SrcVT != MVT::f64)
     return Op;