[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.

llvm-svn: 181532