[RISCV] Hoist duplicate code in lowerINSERT_SUBVECTOR. NFC (#109733)

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -10204,17 +10204,15 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
       Vec = convertToScalableVector(ContainerVT, Vec, DAG, Subtarget);
     }
 
+    SubVec = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, ContainerVT,
+                         DAG.getUNDEF(ContainerVT), SubVec,
+                         DAG.getVectorIdxConstant(0, DL));
+
     if (OrigIdx == 0 && Vec.isUndef() && VecVT.isFixedLengthVector()) {
-      SubVec = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, ContainerVT,
-                           DAG.getUNDEF(ContainerVT), SubVec,
-                           DAG.getVectorIdxConstant(0, DL));
       SubVec = convertFromScalableVector(VecVT, SubVec, DAG, Subtarget);
       return DAG.getBitcast(Op.getValueType(), SubVec);
     }
 
-    SubVec = DAG.getNode(ISD::INSERT_SUBVECTOR, DL, ContainerVT,
-                         DAG.getUNDEF(ContainerVT), SubVec,
-                         DAG.getVectorIdxConstant(0, DL));
     SDValue Mask =
         getDefaultVLOps(VecVT, ContainerVT, DL, DAG, Subtarget).first;
     // Set the vector length to only the number of elements we care about. Note