Generalize register parameter homing to handle float and integer
parameters simultaneously, and to handle all parameters (including the
Swift self register). Base it on the new ABI representation.

The new algorithm constructs a graph in which nodes are the source and
destination registers of all parameters. Edges in the graph indicate
that (part of) a register has to be moved into (part of) another
register. To home parameters we repeatedly pick a register (preferring
nodes without any outgoing edges) and perform the reg-reg moves
indicated by its incoming edges. If we pick a register that has any
outgoing edges it means there is circularity, so we need a temporary
register to save its value.