Avoid Unsafe.As usage in ValueTask that can break type safety (#17471)

Avoid Unsafe.As usage in ValueTask that can break type safety (#17471)

Unsafe.As yields a performance improvement over using a normal cast, and it's fine when ValueTask is used correctly, but if the ValueTask instance were to be stored into a field and multiple threads incorrectly raced to access it, a torn read/write could result in violating type safety due to ObjectIsTask reading the wrong value for the associated object. This commit changes the implementation to only use the single object field to determine which paths to take, rather than factoring in a second field that may not be in sync.