[RISCV] Update comment on vslide*_vl that it has a sixth policy operand. NFC

Author: lukel97

llvm/lib/Target/RISCV/RISCVISelLowering.h

@@ -172,9 +172,9 @@ enum NodeType : unsigned {
   // mask and VL operand.
   TRUNCATE_VECTOR_VL,
   // Matches the semantics of vslideup/vslidedown. The first operand is the
-  // pass-thru operand, the second is the source vector, the third is the
-  // XLenVT index (either constant or non-constant), the fourth is the mask
-  // and the fifth the VL.
+  // pass-thru operand, the second is the source vector, the third is the XLenVT
+  // index (either constant or non-constant), the fourth is the mask, the fifth
+  // is the VL and the sixth is the policy.
   VSLIDEUP_VL,
   VSLIDEDOWN_VL,
   // Matches the semantics of vslide1up/slide1down. The first operand is