[SelectionDAG][VP] Fix MemSDNode::getBasePtr

[frasercrmck]

Summary

Found while working on D108987. When interpreting VP nodes as
MemSDNode nodes, this function would return the incorrect indices.
This was due to VP_GATHER and having no "passthru", and both
VP_GATHER and VP_SCATTER having their mask operands *after* the base
pointer, unlike MGATHER and MSCATTER.

[craig.topper]

LGTM

llvm/include/llvm/CodeGen/SelectionDAGNodes.h

public:
 
   const SDValue &getChain() const { return getOperand(0); }
 
   const SDValue &getBasePtr() const {
     switch (getOpcode()) {
     case ISD::STORE:
     case ISD::VP_STORE:
     case ISD::MSTORE:
+    case ISD::VP_SCATTER:
       return getOperand(2);
     case ISD::MGATHER:
     case ISD::MSCATTER:
-    case ISD::VP_GATHER:
-    case ISD::VP_SCATTER:
       return getOperand(3);
     default:
       return getOperand(1);
     }
   }
 
   // Methods to support isa and dyn_cast
   static bool classof(const SDNode *N) {