Issue Description:

Function signature lowering is the mapping of a function's source-language type, which includes formal parameters and results, all the way down to a physical convention, which dictates what values are stored in what registers and what values to pass on the stack.

Lowering the result value is usually done first, with a certain number of registers designated to hold the result value if it fits, otherwise the result value is passed on the stack. Ahead of ABI stability, a good heuristic is needed for the limit and is architecture specific.