scf.for implicit terminators

[alexarice]

The implicit terminator infrastructure for scf.for does not seem to work correctly at the moment. See #3303

[compor]

@alexarice This is now done, right? I didn't dig deeper but seems the (unrelated) issue that #3303 was trying to solve with printing unregistered ops. If so, close this or please elaborate with a MWE and I'll get around to it.

[alexarice]

%0 = arith.constant 0 : index
scf.for %iv = %0 to %0 step %0 {
  "unregistered"() : () -> ()
}

I would expect this to verify

[alexarice]

For comparison this is ok:

%0 = arith.constant 0 : index
scf.for %iv = %0 to %0 step %0 {
  "test.op"() : () -> ()
}