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/mod.rs

@@ -41,8 +41,8 @@
 //! concurrent function compilations.
 
 pub use isa::encoding::Encoding;
-pub use isa::registers::{RegInfo, RegUnit, RegClass};
-pub use isa::constraints::RecipeConstraints;
+pub use isa::registers::{RegInfo, RegUnit, RegClass, RegClassIndex};
+pub use isa::constraints::{RecipeConstraints, OperandConstraint, ConstraintKind};
 
 use settings;
 use ir::{InstructionData, DataFlowGraph};