Implement value affinities for register allocation.

An SSA value is usually biased towards a specific register class or a
stack slot, depending on the constraints of the instructions using it.

Represent this bias as an Affinity enum, and implement a merging
algorithm for updating an affinity to satisfy a new constraint.

Affinities will be computed as part of the liveness analysis. This is
not implemented yet.

lib/cretonne/src/regalloc/mod.rs

@@ -5,3 +5,5 @@
 pub mod liverange;
 pub mod liveness;
 pub mod allocatable_set;
+
+mod affinity;