[RISCV][GISel] Fix legalizer of G_INSERT_SUBVECTOR (#171091)

Fix early exit condition and use the right type as mask type of
vslideup.

Author: jacquesguan

llvm/lib/Target/RISCV/GISel/RISCVLegalizerInfo.cpp

@@ -1240,7 +1240,7 @@ bool RISCVLegalizerInfo::legalizeInsertSubvector(MachineInstr &MI,
   LLT BigTy = MRI.getType(BigVec);
   LLT LitTy = MRI.getType(LitVec);
 
-  if (Idx == 0 ||
+  if (Idx == 0 &&
       MRI.getVRegDef(BigVec)->getOpcode() == TargetOpcode::G_IMPLICIT_DEF)
     return true;
 
@@ -1314,7 +1314,7 @@ bool RISCVLegalizerInfo::legalizeInsertSubvector(MachineInstr &MI,
   auto Insert = MIB.buildInsertSubvector(InterLitTy, MIB.buildUndef(InterLitTy),
                                          LitVec, 0);
 
-  auto [Mask, _] = buildDefaultVLOps(BigTy, MIB, MRI);
+  auto [Mask, _] = buildDefaultVLOps(InterLitTy, MIB, MRI);
   auto VL = MIB.buildVScale(XLenTy, LitTy.getElementCount().getKnownMinValue());
 
   // If we're inserting into the lowest elements, use a tail undisturbed