Phi nodes in non-header blocks are converted to select instructions after if-conversion. This patch updates the cost model to account for the selects.
We don't need to handle Phi->getNumIncomingValues() == 0 - it doesn't pass the verifier, right?
This returns 1 by default, right? I'm not entirely sure that's the right thing to do here, but it's probably no worse than the current state.
Please also add a CHECK for the induction phi.