Compute register class intersections.

Ensure that the set of register classes is closed under intersection.

Provide a RegClass::intersect() method which finds the register class
representing the intersection of two classes.

Generate a bit-mask of subclasses for each register class to be used by
the intersect() method.

Ensure that register classes are sorted topologically. This is also used
by the intersect() method.

lib/cretonne/src/regalloc/allocatable_set.rs

@@ -120,11 +120,13 @@ mod tests {
     const GPR: RegClass = &RegClassData {
         index: 0,
         width: 1,
+        subclasses: 0,
         mask: [0xf0000000, 0x0000000f, 0],
     };
     const DPR: RegClass = &RegClassData {
         index: 0,
         width: 2,
+        subclasses: 0,
         mask: [0x50000000, 0x0000000a, 0],
     };