Enhance SCEV's brute force loop analysis to handle multiple PHI nodes in the

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;
  }

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@142781 91177308-0d34-0410-b5e6-96231b3b80d8