[Coroutine] Collect CoroBegin if all of terminators are dominated by one coro.destroy

Summary: The original logic seems to be we could collecting a CoroBegin
if one of the terminators could be dominated by one of coro.destroy,
which doesn't make sense.
This patch rewrites the logics to collect CoroBegin if all of
terminators are dominated by one coro.destroy. If there is no such
coro.destroy, we would call hasEscapePath to evaluate if we should
collect it.

Test Plan: check-llvm

Reviewed by: lxfind

Differential Revision: https://reviews.llvm.org/D100614