Fix sext(setcc) -> select_cc using wrong type for setcc.

Also update the comment, since it actually produces a
select (setcc) instead of select_cc.

It was checking and using the setcc result type for the
type of the sext, instead of the type of the compared items.

In my problem case, the sext was to i32 and was used as the setcc type,
but the expected type was i64.

No test since I haven't been able to hit the problem with
this on any in-tree targets.