[LV] Mark induction PHI nodes as allowed exits (NFC).

[rickyz]

Summary

Prior to this change, loop vectorization legality checks only added
induction PHI nodes to AllowedExit (the list of variables which are
allowed to be used outside of the loop) when its SCEV predicates are
alway true. However, this isn't strictly necessary - all necessary
predicates should have held whenever the final value was computed.

This is not expected to cause any functional change. Loop-external IV
uses are already allowed (correctly so, per the above reasoning), since
the existing code never checks induction PHIs against AllowedExit. The
only purpose of this change is to make the code a little easier to
understand by making the contents of AllowedExit consistent with what
the vectorization legality checks allow.

[rickyz]

Remove comment.

[rickyz]

Rebase.

llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

if (ID.getKind() == InductionDescriptor::IK_IntInduction &&
     // Use the phi node with the widest type as induction. Use the last
     // one if there are multiple (no good reason for doing this other
     // than it is expedient). We've checked that it begins at zero and
     // steps by one, so this is a canonical induction variable.
     if (!PrimaryInduction || PhiTy == WidestIndTy)
       PrimaryInduction = Phi;
   }
 
-  // Both the PHI node itself, and the "post-increment" value feeding
-  // back into the PHI node may have external users.
-  // We can allow those uses, except if the SCEVs we have for them rely
-  // on predicates that only hold within the loop, since allowing the exit
-  // currently means re-using this SCEV outside the loop (see PR33706 for more
-  // details).
-  if (PSE.getPredicate().isAlwaysTrue()) {
-    AllowedExit.insert(Phi);
+
+  // Allow external uses of the "post-increment" value feeding back into
+  // the PHI node unless its SCEV relies on predicates that may not hold
+  // outside of the loop (see PR33706 for more details).
+  if (PSE.getPredicate().isAlwaysTrue()) {
     AllowedExit.insert(Phi->getIncomingValueForBlock(TheLoop->getLoopLatch()));
   }
 
   LLVM_DEBUG(dbgs() << "LV: Found an induction variable.\n");
 }
 
 bool LoopVectorizationLegality::setupOuterLoopInductions() {
   BasicBlock *Header = TheLoop->getHeader();