Calculate register active blocks with tradition liveness analysing algorithm

[longfangsong]

Is your feature request related to a problem? Please describe.
Currently, we calculate all_simple_paths from def block to every use block and union them all to find all blocks that a register can be active.

However, traditionally, the liveness analysing algorithm is something like this:

in[exit] = {};
for bb in basic_blocks {
  in[bb] = {};
}
let mut changed = true;
while changed {
  changed = false;
  for bb in basic_blocks {
    out[bb] = bb.succ().map(|s| in[s]).fold(Set::union);
    in[bb] = use[bb] union (out[bb] - def[bb])
  }
}

Which performance is in theory better than calculating all_simple_paths.

However, phi nodes need to be handled in a special way if we want to use this algorithm.

%a = phi [bb1, %0], [bb2, %1] 

It seems it "uses" %0 and %1, but to make the algorithm correct, we should "pretend" %0 is killed by the end of bb1 and %1 is killed by the end of bb2, and they are not in use of this bb.

Describe the solution you'd like
Implement this algorithm and do some benchmarks to make sure we do make a performance improvement.