[Coroutine] Fix a bug where Coroutine incorrectly spills phi and invoke defs before CoroBegin

When a spill definition is before CoroBegin, we cannot spill it to the frame immediately after the definition. We have to spill it after the frame is ready.
The current implementation handles it properly for any other kinds of instructions except for PhINode and InvokeInst, which could also be defined before CoroBegin.
This patch fixes it by moving the CoroBegin dominance check earlier, so that it covers all cases.
Added a test.

Differential Revision: https://reviews.llvm.org/D87810