[LV] Avoid vectorizing first order recurrence when phi uses are outside loop

In the vectorization of first order recurrence, we vectorize such
that the last element in the vector will be the one extracted to pass into the
scalar remainder loop. However, this is not true when there is a phi (other
than the primary induction variable) is used outside the loop.
In such a case, we need the value from the second last iteration (i.e.
the phi value), not the last iteration (which would be the phi update).
I've added a test case for this. Also see PR32396.

A follow up patch would generate the correct code gen for such cases,
and turn this vectorization on.

Differential Revision: https://reviews.llvm.org/D31910

Reviewers: mssimpso
llvm-svn: 299985

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -4069,7 +4069,12 @@ void InnerLoopVectorizer::fixFirstOrderRecurrence(PHINode *Phi) {
   VecPhi->addIncoming(Incoming, LI->getLoopFor(LoopVectorBody)->getLoopLatch());
 
   // Extract the last vector element in the middle block. This will be the
-  // initial value for the recurrence when jumping to the scalar loop.
+  // initial value for the recurrence when jumping to the scalar loop. 
+  // FIXME: Note that the last vector element need not always be the correct one:
+  // consider a loop  where we have phi uses outside the loop - we need the
+  // second last iteration value and not the last one). For now, we avoid
+  // considering such cases as firstOrderRecurrences (see
+  // isFirstOrderRecurrence).
   auto *Extract = Incoming;
   if (VF > 1) {
     Builder.SetInsertPoint(LoopMiddleBlock->getTerminator());