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/isa/constraints.rs

@@ -21,6 +21,7 @@ pub struct OperandConstraint {
 }
 
 /// The different kinds of operand constraints.
+#[derive(Clone, Copy, PartialEq, Eq)]
 pub enum ConstraintKind {
     /// This operand or result must be a register from the given register class.
     Reg,