When dealing with i64 data in function call, such as
incoming argument, return value, passing argument to another
function, GlobalISel splits i64 into 2 i32 in little endian
order. We need to reverse the order since M68k is big endian.
This patch hacks on this by defining our own ValueAssigner.
When spliting arguments, we cache the results. After
the work is done, we store the results back in reverse order.