Avoid a couple theoretical reorderings in ManualResetValueTaskSourceCore (#72657)

Avoid a couple theoretical reorderings in ManualResetValueTaskSourceCore (#72657)

In a typical usage sequence, GetStatus is called and then GetResult is called.  If the read of _result in GetResult could move to before the read of _continuation in GetStatus, then there could be a race condition where _result is read prior to it being written, then _continuation is set as part of the operation completing, and the implementation ends up returning an erroneous result.  Similarly, in SignalCompletion, if the read of _continuationState could moved to before the first read of _continuation, it could be stale.