[SimplifyCFG] accumulate bonus insts cost

SimplifyCFG folds

bool foo() {
  if (cond1) return false;
  if (cond2) return false;
  return true;
}

as

bool foo() {
  if (cond1 | cond2) return false
  return true;
}

'cond2' is called 'bonus insts' in branch folding since they introduce overhead
since the original CFG could do early exit but the folded CFG always executes
them. SimplifyCFG calculates the costs of 'bonus insts' of a folding a BB into
its predecessor BB which shares the destination. If it is below bonus-inst-threshold,
SimplifyCFG will fold that BB into its predecessor and cond2 will always be executed.

When SimplifyCFG calculates the cost of 'bonus insts', it only consider 'bonus' insts
in the current BB to be considered for folding. This causes issue for unrolled loops
which share destinations, e.g.

bool foo(int *a) {
  for (int i = 0; i < 32; i++)
    if (a[i] > 0) return false;
  return true;
}

After unrolling, it becomes

bool foo(int *a) {
  if(a[0]>0) return false
  if(a[1]>0) retur...