Give RegClassData a reference to its parent RegInfo.

This makes it possible to materialize new RegClass references without
requiring a RegInfo reference to be passed around.

- Move the RegInfo::toprc() method to RegClassData.
- Rename RegClassData::intersect() to intersect_index() and provide a
  new intersect() which returns a register class.
- Remove some &RegInfo parameters that are no longer needed.

lib/cretonne/src/regalloc/allocatable_set.rs

@@ -198,7 +198,9 @@ mod tests {
         first: 28,
         subclasses: 0,
         mask: [0xf0000000, 0x0000000f, 0],
+        info: &INFO,
     };
+
     const DPR: RegClass = &RegClassData {
         name: "DPR",
         index: 0,
@@ -208,6 +210,12 @@ mod tests {
         first: 28,
         subclasses: 0,
         mask: [0x50000000, 0x0000000a, 0],
+        info: &INFO,
+    };
+
+    const INFO: RegInfo = RegInfo {
+        banks: &[],
+        classes: &[],
     };
 
     #[test]