[NFC] [Coroutines] Remove unnecessary check and constraints on SmallVector

The CoroSplit pass would check the existence of coroutine intrinsic
before starting work. It is not necessary and wasteful since it would
iterate over the Module.

This patch also removes the constraint on the corresponding of the
SmallVector for the possible coroutines in the Modules. The original
value is 4. Given coroutines is used actually in practice. 4 is really
relatively a low threshold.