aco: improve runtime of live var analysis

The first part of this MR enables ACO's arena allocator for the live variable analysis and the second part of this MR basically implements 'Liveness Sets On Reducible Graphs' from "Computing Liveness Sets for SSA-Form Programs" by F. Brandner et al. The idea is that on reducible CFGs, we can compute the live-out variables in a single pass if we insert the live-ins at loop headers into the live-out sets of the loop's body. Then, we only need one additional pass on loop body blocks to fix the register demand calculation and kill flags.