[VPlan] Don't collect live-ins in collectUsersInExitBlocks. (NFC)

Live-ins don't need to be handled, other than adding to the exit phi
recipe. Do that early and assert that otherwise the exit value is
defined in the vector loop region.

This should enable simply skipping other exit values that do not need
further fixing, e.g. if handling the exit value from the early exit
directly in handleUncountableEarlyExit.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -9052,8 +9052,12 @@ collectUsersInExitBlocks(Loop *OrigLoop, VPRecipeBuilder &Builder,
         }
         Value *IncomingValue = ExitPhi->getIncomingValueForBlock(ExitingBB);
         VPValue *V = Builder.getVPValueOrAddLiveIn(IncomingValue);
-        ExitUsersToFix.insert(ExitIRI);
         ExitIRI->addOperand(V);
+        if (V->isLiveIn())
+          continue;
+        assert(V->getDefiningRecipe()->getParent()->getEnclosingLoopRegion() &&
+               "Only recipes defined inside a region should need fixing.");
+        ExitUsersToFix.insert(ExitIRI);
       }
     }
   }
@@ -9077,11 +9081,6 @@ addUsersInExitBlocks(VPlan &Plan,
   // modeling the corresponding LCSSA phis.
   for (VPIRInstruction *ExitIRI : ExitUsersToFix) {
     for (const auto &[Idx, Op] : enumerate(ExitIRI->operands())) {
-      // Pass live-in values used by exit phis directly through to their users
-      // in the exit block.
-      if (Op->isLiveIn())
-        continue;
-
       // Currently only live-ins can be used by exit values from blocks not
       // exiting via the vector latch through to the middle block.
       if (ExitIRI->getParent()->getSinglePredecessor() != MiddleVPBB)