Introduce ad hoc liveness tracking utility: LiveRegUnits

Contains a set of live register (units) and code to move forward and
backward in the schedule while updating the live set.
Remove kill flags after if conversion if necessary

When if converting something like:
true:
   ... = R0<kill>

false:
   ... = R0<kill>

then the instructions of the true block must not have a <kill> flag
anymore, as the instruction of the false block follow and do still read
the R0 value.
Specifically this patch determines the set of register live-in in the
false block (possibly after simulating the liveness changes of the
duplicated instructions). Each of these live-in registers mustn't be
killed.