Slow reduction using delta debugging

I see

|input size: 248089                         
|starting reduction!                        
|    try to remove function bodies          
|     try partition 1 / 1 (size 0 / 1745)   
|     try partition 1 / 2 (size 873 / 1745) 
|     try partition 2 / 2 (size 872 / 1745) 
|     try partition 1 / 4 (size 437 / 1745) 
|     try partition 2 / 4 (size 436 / 1745) 
|     try partition 3 / 4 (size 436 / 1745) 
|     try partition 4 / 4 (size 436 / 1745)  
|     try partition 1 / 4 (size 1308 / 1745) 
|     try partition 2 / 4 (size 1309 / 1745) 
|     try partition 3 / 4 (size 1309 / 1745) 
|     try partition 4 / 4 (size 1309 / 1745) 
|     try partition 1 / 8 (size 219 / 1745) 
|     try partition 2 / 8 (size 218 / 1745) 
|     try partition 3 / 8 (size 218 / 1745) 
|     try partition 4 / 8 (size 218 / 1745) 
|     try partition 5 / 8 (size 218 / 1745) 
|     try partition 6 / 8 (size 218 / 1745) 
|     try partition 7 / 8 (size 218 / 1745) 
|     try partition 8 / 8 (size 218 / 1745) 
|     try partition 1 / 8 (size 1526 / 1745)
|     try partition 2 / 8 (size 1527 / 1745) 
|     try partition 3 / 8 (size 1527 / 1745) 
|     try partition 1 / 7 (size 219 / 1527)  
|     try partition 2 / 7 (size 218 / 1527)  
|     try partition 3 / 7 (size 218 / 1527)
|     try partition 4 / 7 (size 218 / 1527)
|     try partition 5 / 7 (size 218 / 1527)
|     try partition 6 / 7 (size 218 / 1527)
|     try partition 7 / 7 (size 218 / 1527)
|     try partition 1 / 7 (size 1308 / 1527)
|     try partition 2 / 7 (size 1309 / 1527)
|     try partition 3 / 7 (size 1309 / 1527)
|     try partition 4 / 7 (size 1309 / 1527)
|     try partition 5 / 7 (size 1309 / 1527)
|     try partition 1 / 6 (size 219 / 1309)
|     try partition 2 / 6 (size 218 / 1309)
|     try partition 3 / 6 (size 218 / 1309)
|     try partition 4 / 6 (size 218 / 1309)

And it just keeps trying more and more here. This has been doing on for several minutes, with each change taking 1-2 seconds. It is making progress, but trying the passes and destructive reduction etc. in between could be a lot faster, I think.

@tlively I think we do need something like taking the factor into account. The basic idea there is that no single method works to exhaustion, they each try a small amount and then cycle. No single method knows how well the others can do - e.g. possibly a single -O3 would immediately do all the work that function removal is doing manually.

Start by reproducing the delta-debugging reduction described in the issue and inspect the repeated partition attempts in the reduction process. Compare the time spent on function removal with the intervening passes and destructive reduction; done means reducing unnecessary repeated work while preserving effective reduction progress.