[RISCV][NFC] Remove ISel of SPLAT_VECTOR

Since we have converted SPLAT_VECTOR to VMV_V_X_VL
or VFMV_V_F_VL in RISCVDAGToDAGISel::PreprocessISelDAG().

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D136814