Fix checking the vector size instead of the element size

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp

@@ -3305,8 +3305,7 @@ bool RISCVDAGToDAGISel::selectLow8BitsVSplat(SDValue N, SDValue &SplatVal) {
 
   // We can have multiple nested nodes, so unravel them all if needed.
   while (IsExtOrTrunc(N)) {
-    if (!N.hasOneUse() ||
-        N.getValueType().getSizeInBits().getKnownMinValue() < 8)
+    if (!N.hasOneUse() || N.getScalarValueSizeInBits() < 8)
       return false;
     N = N->getOperand(0);
   }