[Coroutines] Don't merge readnone calls in presplit coroutines

Another alternative to fix the thread identification problem in
coroutines.

We plan to fix this problem by unifying memory effecting attributes. See
https://discourse.llvm.org/t/rfc-unify-memory-effect-attributes/65579.
But it may be a long-term project. And it is a pity that the coroutines
can't resume in different threads for years. So this one is temporary
fix. It may cause unnecessary performance regression for coroutines. But
correctness are more important. And this one is planned to be reverted
after we are able to unify the memory effecting attributes actually.

Reviewed By: jdoerfert, rjmccall

Differential Revision: https://reviews.llvm.org/D135550