The Real Clear Politics poll average might support the inference that 6 percentage points of Cruz's final number were people who would have gone for Kasich if they had not been motivated by the desire to stop Trump. Click to enlarge:
Trump got what the polls predicted, about 35%. But Kasich got 6 points less than the predicted 20%. Cruz ended up with 9 points more than what the polls predicted, so it seems as though what Kasich lost, Cruz gained. Cruz picked up even more, presumably the previously undecided.