[Linalg] Fix a warning

This patch fixes:

  mlir/lib/Dialect/Linalg/Transforms/Vectorization.cpp:821:12: error:
  variable 'countNonUnitDim' set but not used
  [-Werror,-Wunused-but-set-variable]

Author: kazutakahirata

mlir/lib/Dialect/Linalg/Transforms/Vectorization.cpp

@@ -829,6 +829,7 @@ static uint64_t getNonUnitLoopDim(LinalgOp linalgOp) {
   assert(linalgOp.hasDynamicShape() ||
          countNonUnitDim == 1 && "For statically shaped Linalg Ops, only one "
                                  "non-unit loop dim is expected");
+  (void)countNonUnitDim;
   return nonUnitDim;
 }