[MLIR] Fix warning: variable used uninitialized. (#4337)

Co-authored-by: Robert Kimball <robert.kimball@intel.com>
Co-authored-by: Sang Ik Lee <sang.ik.lee@intel.com>

src/contrib/mlir/backend/pass/affine_lowerer.cpp

@@ -2112,6 +2112,10 @@ namespace
         {
             ty = OpType::MAXPOOL;
         }
+        else
+        {
+            NGRAPH_UNREACHABLE("Unsupported pooling op");
+        }
 
         auto unrankedMemrefTy = UnrankedMemRefType::get(elemTy, 0);
         SmallVector<mlir::Value, 4> inputs = {lhs, result};