[LV] Use VPlan pattern matching in adjustRecipesForReductions (NFC)

Replace the assert checking if CurrentLinkI is a CmpInst with a pattern
matching check in the if condition. This uses VPlan-level pattern matching
instead of inspecting the underlying instruction type.

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8727,11 +8727,8 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
         VecOp = Sub;
       } else {
         if (RecurrenceDescriptor::isMinMaxRecurrenceKind(Kind)) {
-          if (isa<VPWidenRecipe>(CurrentLink)) {
-            assert(isa<CmpInst>(CurrentLinkI) &&
-                   "need to have the compare of the select");
+          if (match(CurrentLink, m_Cmp(m_VPValue(), m_VPValue())))
             continue;
-          }
           assert(isa<VPWidenSelectRecipe>(CurrentLink) &&
                  "must be a select recipe");
           IndexOfFirstOperand = 1;

llvm/lib/Transforms/Vectorize/VPlanPatternMatch.h

@@ -29,6 +29,10 @@ template <typename Pattern> bool match(VPUser *U, const Pattern &P) {
   return R && match(R, P);
 }
 
+template <typename Pattern> bool match(VPSingleDefRecipe *R, const Pattern &P) {
+  return P.match(static_cast<const VPRecipeBase *>(R));
+}
+
 template <typename Val, typename Pattern> struct VPMatchFunctor {
   const Pattern &P;
   VPMatchFunctor(const Pattern &P) : P(P) {}