[NFC] Use initial-stack-allocations for some nested containers

I've done a little more testing on these as, being nested, there's more
initialization to do when they're created. The net effect is positive on
the compile-time-tracker.

llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp

@@ -18005,7 +18005,7 @@ class HorizontalReduction {
   /// List of possibly reduced values.
   SmallVector<SmallVector<Value *>> ReducedVals;
   /// Maps reduced value to the corresponding reduction operation.
-  DenseMap<Value *, SmallVector<Instruction *>> ReducedValsToOps;
+  SmallDenseMap<Value *, SmallVector<Instruction *>, 16> ReducedValsToOps;
   WeakTrackingVH ReductionRoot;
   /// The type of reduction operation.
   RecurKind RdxKind;
@@ -18374,7 +18374,9 @@ class HorizontalReduction {
     // instruction op id and/or alternate op id, plus do extra analysis for
     // loads (grouping them by the distabce between pointers) and cmp
     // instructions (grouping them by the predicate).
-    MapVector<size_t, MapVector<size_t, MapVector<Value *, unsigned>>>
+    SmallMapVector<
+        size_t, SmallMapVector<size_t, SmallMapVector<Value *, unsigned, 2>, 2>,
+        8>
         PossibleReducedVals;
     initReductionOps(Root);
     DenseMap<Value *, SmallVector<LoadInst *>> LoadsMap;