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