[X86] Fix an unused variable warning (NFC)

/llvm-project/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp:3090:7:
 error: unused variable 'IndexType' [-Werror,-Wunused-variable]
  EVT IndexType = Index.getValueType();
      ^
1 error generated.
diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index f1692b5..c8ca575 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -3087,7 +3087,7 @@
     Base = CurDAG->getTargetInsertSubreg(SubReg, DL, MVT::i64, ImplDef, Base);
   }
 
-  EVT IndexType = Index.getValueType();
+  [[maybe_unused]] EVT IndexType = Index.getValueType();
   RN = dyn_cast<RegisterSDNode>(Index);
   if (RN && RN->getReg() == 0)
     Index = CurDAG->getRegister(0, MVT::i64);