Topologically sort 'copy' commands.

Topologically sort 'copy' commands.

The order in which copy commands are executed is important:
'x = y; y = z;' is not the same as 'y = z; x = y;' (the latter
overwrites 'y' before its precious value is copied to 'x').
To avoid overwrites, commands should be topologically sorted.
This is always possible because there's no cyclic dependencies
by construction.

Found by slyfox's fuzzer. :)