Fixing vector add pattern that incorrectly returns success.

[benvanik]

Summary

The pattern is returning success even if it does no work leading to pattern application running up to the max iteration count and failing.

[nicolasvasilache]

thanks @benvanik !

mlir/lib/Dialect/Vector/VectorOps.cpp

     Value a = addOp->getOperand(0), b = addOp->getOperand(1);
     vector::ContractionOp contract = canonicalize(a, b);
     contract = contract ? contract : canonicalize(b, a);
-    return success();
+    return contract ? success() : failure();
   }
 };