Correct an assertion string.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@54348 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/SelectionDAGNodes.h

@@ -1097,7 +1097,7 @@ public:
   /// true. It returns the MachineInstr opcode value that the node's opcode
   /// corresponds to.
   unsigned getMachineOpcode() const {
-    assert(isMachineOpcode() && "Not a target opcode!");
+    assert(isMachineOpcode() && "Not a MachineInstr opcode!");
     return ~NodeType;
   }