After finding a register solution, it need to be executed as a sequence
of regmove instructions. This often requires a topological ordering of
the moves so they don't conflict.
When the solution contains cycles, try to grab an available scratch
register to implement the copies. Panic if that fails (later, we'll
implement emergency spilling in this case).
Make sure we handle odd aliasing in the arm32 floating point register
bank. Not everything is a simple cycle in that case, so make sure we
don't assume so.
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