message: memory: minor code/expr simplification
  (most allocations succeed)
diff: @@ -505,11 +505,7 @@ expr Memory::alloc(const expr &size, unsigned align, BlockKind blockKind,
  local_blk_size.add(expr(short_bid), move(size_zext));
  local_blk_kind.add(expr(short_bid), expr::mkUInt(alloc_ty, 2));
  - local_block_liveness
  - = expr::mkIf(allocated,
  - local_block_liveness.store(short_bid, true),
  - local_block_liveness);
  -
  + local_block_liveness = local_block_liveness.store(short_bid, allocated);
  state->addPre(allocated.implies(p.get_address() != 0));
  } else {
  