[PATCH] [Jump-Threading] Fixed jump threading hang issues (PR15386, PR15851)

[Dinesh Dwivedi]

Each pass iterates over all BBs of the functions and all updated BBs are added to 
'UpdatedBB' so if first pass adds 3 BBs and second pass adds 2, there will be 
5 BBs in the list and we will still continue with next pass.

Every change in BBs actually adds atleast one new block XXX.thread and till it finds 
some new block added or updated, it will do next iteration on all BBs of the function.
and the condition, that will break this is when we are not adding any new block or not
updatind any new BB.

In attached test case, jump threading stuck between 'for.cond1' and 'for.body' block.
Here is sample output for few iterations.
After first iteration:
  for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
    %div5 = phi i32 [ 0, %for.body.thread ], [ %div, %for.body ], [ %div, %land.rhs ]
    %inc = add nsw i32 %div5, 1
    %cmp = icmp slt i32 %inc, 2
    br i1 %cmp, label %for.body, label %for.cond1.thread

  for.cond1.thread:                                 ; preds = %for.cond1
    br label %for.body.thread

  for.body.thread:                                  ; preds = %for.cond1.thread
    br label %for.cond1

  for.body:                                         ; preds = %for.cond1
    %i.02 = phi i32 [ %inc, %for.cond1 ]
    %div = sdiv i32 %i.02, 2
    %i.0.off = add i32 %i.02, 1
    %0 = icmp ugt i32 %i.0.off, 2
    br i1 %0, label %land.rhs, label %for.cond1

2nd:
  for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
    %div5 = phi i32 [ %div, %for.body ], [ %div, %land.rhs ], [ 0, %for.body.thread ]
    %inc = add nsw i32 %div5, 1
    %cmp = icmp slt i32 %inc, 2
    br i1 %cmp, label %for.body, label %for.cond1.thread6

  for.cond1.thread6:                                ; preds = %for.cond1
    br label %for.body.thread

  for.body.thread:                                  ; preds = %for.cond1.thread6
    br label %for.cond1

  for.body:                                         ; preds = %for.cond1
    %i.02 = phi i32 [ %inc, %for.cond1 ]
    %div = sdiv i32 %i.02, 2
    %i.0.off = add i32 %i.02, 1
    %0 = icmp ugt i32 %i.0.off, 2
    br i1 %0, label %land.rhs, label %for.cond1

and 3rd:
  for.cond1:                                        ; preds = %for.body.thread, %for.body, %land.rhs
    %div5 = phi i32 [ %div, %for.body ], [ %div, %land.rhs ], [ 0, %for.body.thread ]
    %inc = add nsw i32 %div5, 1
    %cmp = icmp slt i32 %inc, 2
    br i1 %cmp, label %for.body, label %for.cond1.thread

  for.cond1.thread:                                 ; preds = %for.cond1
    br label %for.body.thread

  for.body.thread:                                  ; preds = %for.cond1.thread
    br label %for.cond1

  for.body:                                         ; preds = %for.cond1
    %i.02 = phi i32 [ %inc, %for.cond1 ]
    %div = sdiv i32 %i.02, 2
    %i.0.off = add i32 %i.02, 1
    %0 = icmp ugt i32 %i.0.off, 2
    br i1 %0, label %land.rhs, label %for.cond1

I will update test cases as you suggested. Actually I am not sure about test as 
it hangs with trunk and check-all never completes.

http://reviews.llvm.org/D3991