Fixes#165.
The constraint solver's schedule_move() function sometimes need to use
an extra available register when the moves to be scheduled contains
cycles.
The pending moves have associated register classes that come from the
constraint programming. Since the moves have hard-coded to and from
registers, these register classes are only meant to indicate the
register sizes. In particular, we can use the whole top-level register
class when scavenging for a spare register to break a cycle.
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