[RISCV] Treat insert_subvector into undef with index==0 as legal. (#109745)

Regardless of fixed and scalable type. We can always use subreg ops.

We don't need to do any container conversion.

Author: topperc

llvm/lib/Target/RISCV/RISCVISelLowering.cpp

@@ -10152,13 +10152,15 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
   unsigned OrigIdx = Op.getConstantOperandVal(2);
   const RISCVRegisterInfo *TRI = Subtarget.getRegisterInfo();
 
+  if (OrigIdx == 0 && Vec.isUndef())
+    return Op;
+
   // We don't have the ability to slide mask vectors up indexed by their i1
   // elements; the smallest we can do is i8. Often we are able to bitcast to
   // equivalent i8 vectors. Note that when inserting a fixed-length vector
   // into a scalable one, we might not necessarily have enough scalable
   // elements to safely divide by 8: nxv1i1 = insert nxv1i1, v4i1 is valid.
-  if (SubVecVT.getVectorElementType() == MVT::i1 &&
-      (OrigIdx != 0 || !Vec.isUndef())) {
+  if (SubVecVT.getVectorElementType() == MVT::i1) {
     if (VecVT.getVectorMinNumElements() >= 8 &&
         SubVecVT.getVectorMinNumElements() >= 8) {
       assert(OrigIdx % 8 == 0 && "Invalid index");
@@ -10196,8 +10198,6 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
   // vector group up the full amount.
   const auto VLen = Subtarget.getRealVLen();
   if (SubVecVT.isFixedLengthVector() && !VLen) {
-    if (OrigIdx == 0 && Vec.isUndef() && !VecVT.isFixedLengthVector())
-      return Op;
     MVT ContainerVT = VecVT;
     if (VecVT.isFixedLengthVector()) {
       ContainerVT = getContainerForFixedLengthVector(VecVT);
@@ -10208,11 +10208,6 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
                          DAG.getUNDEF(ContainerVT), SubVec,
                          DAG.getVectorIdxConstant(0, DL));
 
-    if (OrigIdx == 0 && Vec.isUndef() && VecVT.isFixedLengthVector()) {
-      SubVec = convertFromScalableVector(VecVT, SubVec, DAG, Subtarget);
-      return DAG.getBitcast(Op.getValueType(), SubVec);
-    }
-
     SDValue Mask =
         getDefaultVLOps(VecVT, ContainerVT, DL, DAG, Subtarget).first;
     // Set the vector length to only the number of elements we care about. Note