Speculatively revert commits 142790 and 142843 to see if it fixes

Speculatively revert commits 142790 and 142843 to see if it fixes
the dragonegg and llvm-gcc self-host buildbots.  Original commit
messages:
- Reapply r142781 with fix. Original message:

  Enhance SCEV's brute force loop analysis to handle multiple PHI nodes in the
  loop header when computing the trip count.

  With this, we now constant evaluate:
    struct ListNode { const struct ListNode *next; int i; };
    static const struct ListNode node1 = {0, 1};
    static const struct ListNode node2 = {&node1, 2};
    static const struct ListNode node3 = {&node2, 3};
    int test() {
      int sum = 0;
      for (const struct ListNode *n = &node3; n != 0; n = n->next)
        sum += n->i;
      return sum;
    }

- Now that we look at all the header PHIs, we need to consider all the header PHIs
when deciding that the loop has stopped evolving. Fixes miscompile in the gcc
torture testsuite!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142916 91177308-0d34-0410-b5e6-96231b3b80d8