[VPlan] Only skip induction phis in planContainsAdditionalSimps (NFC).

Skip induction phis when checking for simplifications, as they may not
be lowered directly be lowered to a corresponding PHI recipe. Reductions
and first-order recurrences will get lowered to phi recipes, unless they
are removed. Considering them for simplifications allows removing them
if there are no remaining users.

NFC as currently reduction and recurrence phis are not
simplified/removed if dead.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -7061,7 +7061,10 @@ static bool planContainsAdditionalSimplifications(VPlan &Plan,
   return any_of(TheLoop->blocks(), [&SeenInstrs, &CostCtx,
                                     TheLoop](BasicBlock *BB) {
     return any_of(*BB, [&SeenInstrs, &CostCtx, TheLoop, BB](Instruction &I) {
-      if (isa<PHINode>(&I) && BB == TheLoop->getHeader())
+      // Skip induction phis when checking for simplifications, as they may not
+      // be lowered directly be lowered to a corresponding PHI recipe.
+      if (isa<PHINode>(&I) && BB == TheLoop->getHeader() &&
+          CostCtx.CM.Legal->isInductionPhi(cast<PHINode>(&I)))
         return false;
       return !SeenInstrs.contains(&I) && !CostCtx.skipCostComputation(&I, true);
     });