[SimpleLoopBoundSplit] Split Bound of Loop which has conditional branch with IV

[SimpleLoopBoundSplit] Split Bound of Loop which has conditional branch with IV

This pass transforms loops that contain a conditional branch with induction
variable. For example, it transforms left code to right code:

                             newbound = min(n, c)
 while (iv < n) {            while(iv < newbound) {
   A                           A
   if (iv < c)                 B
     B                         C
   C                         }
 }                           if (iv != n) {
                               while (iv < n) {
                                 A
                                 C
                               }
                             }

Differential Revision: https://reviews.llvm.org/D102234