add Sethi-Ullman (a.k.a. just DP it)
use $tX first and use stack instead of queue (not round-Robin anymore)
save register only when it was used
main should not call exit; only the runtime system should
change `enstack`: throw SReg the end
save used SRegs only (forgot to mark visited)
revert `enstack` modification
calculate ArrayRef constants
MIPSTest should check compile success
block sorting and jump elimination