nir/live_variables: Use a worklist

nir/live_variables: Use a worklist

This is a rework of the liveness algorithm using a worklist as suggested by
Connor.  Doing so reduces the number of times we walk over the instructions
because we don't have to do an entire pointless walk over the instructions
just to figure out it's time to stop.  Also, the stuff after the last loop
in the funciton will only ever get visited once.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>