Define the AffineForOp and replace ForInst with it. This patch is largely...

Define the AffineForOp and replace ForInst with it. This patch is largely mechanical, i.e. changing usages of ForInst to OpPointer<AffineForOp>. An important difference is that upon construction an AffineForOp no longer automatically creates the body and induction variable. To generate the body/iv, 'createBody' can be called on an AffineForOp with no body.

PiperOrigin-RevId: 232060516