[RISCV] Fix incorrect codegen introduced by D119688.

We should not emit a tail agnostic vlse for a tail undisturbed vmv.s.x

In D119688:
-    if (IsScalarMove && !Node->getOperand(0).isUndef())
+    bool HasPassthruOperand = Node->getOpcode() != ISD::SPLAT_VECTOR;
+    if (HasPassthruOperand && !IsScalarMove &&
!Node->getOperand(0).isUndef())
       break;

The IsScalarMove check in the if statement had been changed.

Differential Revision: https://reviews.llvm.org/D120963