LoopVectorizer: Disallow reductions whose header phi is used outside the loop

If an outside loop user of the reduction value uses the header phi node we
cannot just reduce the vectorized phi value in the vector code epilog because
we would loose VF-1 reductions.

lp:
  p = phi (0, lv)
  lv = lv + 1
  ...
  brcond , lp, outside

outside:
  usr = add 0, p

(Say the loop iterates two times, the value of p coming out of the loop is one).

We cannot just transform this to:

vlp:
  p = phi (<0,0>, lv)
  lv = lv + <1,1>
  ..
  brcond , lp, outside

outside:
  p_reduced = p[0] + [1];
  usr = add 0, p_reduced

(Because the original loop iterated two times the vectorized loop would iterate
one time, but p_reduced ends up being zero instead of one).

We would have to execute VF-1 iterations in the scalar remainder loop in such
cases. For now, just disable vectorization.

PR16522

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@186256 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -3818,9 +3818,13 @@ bool LoopVectorizationLegality::AddReductionVar(PHINode *Phi,
       // Check if we found the exit user.
       BasicBlock *Parent = Usr->getParent();
       if (!TheLoop->contains(Parent)) {
-        // Exit if you find multiple outside users.
-        if (ExitInstruction != 0)
+        // Exit if you find multiple outside users or if the header phi node is
+        // being used. In this case the user uses the value of the previous
+        // iteration, in which case we would loose "VF-1" iterations of the
+        // reduction operation if we vectorize.
+        if (ExitInstruction != 0 || Cur == Phi)
           return false;
+
         ExitInstruction = Cur;
         continue;
       }