[coroutines] Improve suspend point simplification

Summary:
Enable suspend point simplification for cases where:
* coro.save and coro.suspend are in different basic blocks
* where there are intervening intrinsics

Reviewers: modocache, tks2103, lewissbaker

Reviewed By: modocache

Subscribers: EricWF, llvm-commits

Differential Revision: https://reviews.llvm.org/D55160

llvm-svn: 348897