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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192481 91177308-0d34-0410-b5e6-96231b3b80d8
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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192482 91177308-0d34-0410-b5e6-96231b3b80d8