[SLP] reduce code for finding reduction costs; NFC

We can get both (vector/scalar) costs in a single switch
instead of sequentially.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -7140,6 +7140,7 @@ class HorizontalReduction {
     RecurKind Kind = RdxTreeInst.getKind();
     unsigned RdxOpcode = RecurrenceDescriptor::getOpcode(Kind);
     int SplittingRdxCost;
+    int ScalarReduxCost;
     switch (Kind) {
     case RecurKind::Add:
     case RecurKind::Mul:
@@ -7150,6 +7151,7 @@ class HorizontalReduction {
     case RecurKind::FMul:
       SplittingRdxCost = TTI->getArithmeticReductionCost(
           RdxOpcode, VecTy, /*IsPairwiseForm=*/false);
+      ScalarReduxCost = TTI->getArithmeticInstrCost(RdxOpcode, ScalarTy);
       break;
     case RecurKind::SMax:
     case RecurKind::SMin:
@@ -7160,42 +7162,21 @@ class HorizontalReduction {
       SplittingRdxCost =
           TTI->getMinMaxReductionCost(VecTy, VecCondTy,
                                       /*IsPairwiseForm=*/false, IsUnsigned);
-      break;
-    }
-    default:
-      llvm_unreachable("Expected arithmetic or min/max reduction operation");
-    }
-
-    int ScalarReduxCost = 0;
-    switch (Kind) {
-    case RecurKind::Add:
-    case RecurKind::Mul:
-    case RecurKind::Or:
-    case RecurKind::And:
-    case RecurKind::Xor:
-    case RecurKind::FAdd:
-    case RecurKind::FMul:
-      ScalarReduxCost = TTI->getArithmeticInstrCost(RdxOpcode, ScalarTy);
-      break;
-    case RecurKind::SMax:
-    case RecurKind::SMin:
-    case RecurKind::UMax:
-    case RecurKind::UMin:
       ScalarReduxCost =
           TTI->getCmpSelInstrCost(RdxOpcode, ScalarTy) +
           TTI->getCmpSelInstrCost(Instruction::Select, ScalarTy,
                                   CmpInst::makeCmpResultType(ScalarTy));
       break;
+    }
     default:
       llvm_unreachable("Expected arithmetic or min/max reduction operation");
     }
-    ScalarReduxCost *= (ReduxWidth - 1);
 
+    ScalarReduxCost *= (ReduxWidth - 1);
     LLVM_DEBUG(dbgs() << "SLP: Adding cost "
                       << SplittingRdxCost - ScalarReduxCost
                       << " for reduction that starts with " << *FirstReducedVal
                       << " (It is a splitting reduction)\n");
-
     return SplittingRdxCost - ScalarReduxCost;
   }