Compute top-level register classes for each register bank.

A top-level register class is one that has no sub-classes. It is
possible to have multiple top-level register classes in the same
register bank. For example, ARM's FPR bank has both D and Q top-level
register classes.

Number register classes such that all top-level register classes appear
as a contiguous sequence starting from 0. This will be used by the
register allocator when counting used registers per top-level register
class.

lib/cretonne/src/regalloc/allocatable_set.rs

@@ -141,6 +141,7 @@ mod tests {
         index: 0,
         width: 1,
         bank: 0,
+        toprc: 0,
         first: 28,
         subclasses: 0,
         mask: [0xf0000000, 0x0000000f, 0],
@@ -150,6 +151,7 @@ mod tests {
         index: 0,
         width: 2,
         bank: 0,
+        toprc: 0,
         first: 28,
         subclasses: 0,
         mask: [0x50000000, 0x0000000a, 0],