Specify priority of register selection within phases of register allocation. Highest priority
is first. A useful heuristic is to give registers a low priority when they are required by
machine instructions, like EAX and EDX on I486, and choose no-save registers before
save-on-call, & save-on-call before save-on-entry. Registers which participate in fixed
calling sequences should come last. Registers which are used as pairs must fall on an even
Adopted from x86_64.ad.