[GVN] Split critical-edge on the fly, instead of postpone edge-splitting to next
  iteration.
  
  This on step toward non-iterative GVN. My local hack suggests that getting rid
of iteration will speedup GVN by 30%+ on a medium sized input (2k LOC, C++).
I cannot explain why not 2x or more at this moment.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181532 91177308-0d34-0410-b5e6-96231b3b80d8