[SLP] Reduce scope of variable using if clause [NFC]

This particular variable name is shadowed by another lower in the
function, so reducing it's scope to it's single use removes the
shadowing and makes the code much less error prone.

Author: preames

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -17579,14 +17579,14 @@ class HorizontalReduction {
     // If there are a sufficient number of reduction values, reduce
     // to a nearby power-of-2. We can safely generate oversized
     // vectors and rely on the backend to split them to legal sizes.
-    unsigned NumReducedVals =
-        std::accumulate(ReducedVals.begin(), ReducedVals.end(), 0,
-                        [](unsigned Num, ArrayRef<Value *> Vals) -> unsigned {
-                          if (!isGoodForReduction(Vals))
-                            return Num;
-                          return Num + Vals.size();
-                        });
-    if (NumReducedVals < ReductionLimit &&
+    if (unsigned NumReducedVals = std::accumulate(
+            ReducedVals.begin(), ReducedVals.end(), 0,
+            [](unsigned Num, ArrayRef<Value *> Vals) -> unsigned {
+              if (!isGoodForReduction(Vals))
+                return Num;
+              return Num + Vals.size();
+            });
+        NumReducedVals < ReductionLimit &&
         (!AllowHorRdxIdenityOptimization ||
          all_of(ReducedVals, [](ArrayRef<Value *> RedV) {
            return RedV.size() < 2 || !allConstant(RedV) || !isSplat(RedV);