Mark maybe_unused variable (#133069)

... to avoid -Wunused-variable warnings/errors when assertions are off.

Author: googlewalt

llvm/lib/Transforms/Vectorize/VPlanRecipes.cpp

@@ -619,7 +619,7 @@ Value *VPInstruction::generate(VPTransformState &State) {
     auto *PhiR = cast<VPReductionPHIRecipe>(getOperand(0));
     // Get its reduction variable descriptor.
     const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
-    RecurKind RK = RdxDesc.getRecurrenceKind();
+    [[maybe_unused]] RecurKind RK = RdxDesc.getRecurrenceKind();
     assert(RecurrenceDescriptor::isFindLastIVRecurrenceKind(RK) &&
            "Unexpected reduction kind");
     assert(!PhiR->isInLoop() &&