SSA register coloring pass.

This is a bare-bones outline of the SSA coloring pass. Many features are
missing, including:

- Handling instruction operand constraints beyond simple register
  classes.
- Handling ABI requirements for function arguments and return values.
- Generating shuffle code for EBB arguments.

lib/cretonne/src/isa/constraints.rs

@@ -49,6 +49,7 @@ pub enum ConstraintKind {
 }
 
 /// Constraints for an encoding recipe.
+#[derive(Clone)]
 pub struct RecipeConstraints {
     /// Constraints for the instruction's fixed value operands.
     ///