[CodeGen] Fix -Wunused-variable in SelectionDAG.cpp (NFC)

Summary:
/llvm-project/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp:7560:9:
error: unused variable 'VecVT' [-Werror,-Wunused-variable]
    EVT VecVT = N1.getValueType();
        ^
1 error generated.

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Differential Revision: https://phabricator.intern.facebook.com/D60251014

Author: DamonFool

llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

@@ -7557,7 +7557,7 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       return N1;
     break;
   case ISD::VECTOR_COMPRESS: {
-    EVT VecVT = N1.getValueType();
+    [[maybe_unused]] EVT VecVT = N1.getValueType();
     [[maybe_unused]] EVT MaskVT = N2.getValueType();
     [[maybe_unused]] EVT PassthruVT = N3.getValueType();
     assert(VT == VecVT && "Vector and result type don't match.");