[Vectorize] Fix a warning

This patch fixes:

  llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp:726:13: error:
  unused variable 'RedPhiRK' [-Werror,-Wunused-variable]

Author: kazutakahirata

llvm/lib/Transforms/Vectorize/VPlanConstruction.cpp

@@ -726,6 +726,7 @@ bool VPlanTransforms::handleMaxMinNumReductionsWithoutFastMath(VPlan &Plan) {
   RecurKind RedPhiRK = RedPhiR->getRecurrenceKind();
   assert((RedPhiRK == RecurKind::FMaxNum || RedPhiRK == RecurKind::FMinNum) &&
          "unsupported reduction");
+  (void)RedPhiRK;
 
   /// Check if the vector loop of \p Plan can early exit and restart
   /// execution of last vector iteration in the scalar loop. This requires all