[LV] Use MapVector for InstsToScalarize for deterministic iter order (NFC)

We iterate over InstsToScalarize when printing costs, and currently the
iteration order is not deterministic. Currently no tests check the
output with multiple instructions in InstsToScalarize, but those will
come soon.

(cherry picked from commit c300a99)

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1596,7 +1596,7 @@ class LoopVectorizationCostModel {
   /// presence of a cost for an instruction in the mapping indicates that the
   /// instruction will be scalarized when vectorizing with the associated
   /// vectorization factor. The entries are VF-ScalarCostTy pairs.
-  DenseMap<ElementCount, ScalarCostsTy> InstsToScalarize;
+  MapVector<ElementCount, ScalarCostsTy> InstsToScalarize;
 
   /// Holds the instructions known to be uniform after vectorization.
   /// The data is collected per VF.