Fix adding dummy states in index reduction (#7625)

- Change case in makeAllDummyVarandDummyDerivativeRepl1 to check for
  StateSelect.Prefer like the comment says.

OMCompiler/Compiler/BackEnd/BackendVariable.mo

@@ -515,6 +515,17 @@ algorithm
   end match;
 end varStateSelectNever;
 
+public function varStateSelectPrefer
+  "Returns true, if the state select attribute is DAE.PREFER()"
+  input BackendDAE.Var inVar;
+  output Boolean isPrefer;
+algorithm
+  isPrefer := match(varStateSelect(inVar))
+    case DAE.PREFER() then true;
+    else false;
+  end match;
+end varStateSelectPrefer;
+
 public function setVarStateSelect "Sets the state select attribute of a variable."
   input BackendDAE.Var inVar;
   input DAE.StateSelect stateSelect;

OMCompiler/Compiler/BackEnd/IndexReduction.mo

@@ -3808,7 +3808,7 @@ algorithm
       then (BackendDAE.VAR(name,BackendDAE.DUMMY_STATE(),dir,prl,tp,bind,tplExp,dim,source,attr,ts,hideResult,comment,ct,io,false),(vars,so,varlst,ht));
     // regular variable with StateSelect.Prefer
     case (var as BackendDAE.VAR(name,BackendDAE.VARIABLE(),dir,prl,tp,bind,tplExp,dim,source,attr,ts,hideResult,comment,ct,io),(vars,so,varlst,ht))
-      guard(BackendVariable.varStateSelectNever(var))
+      guard(BackendVariable.varStateSelectPrefer(var))
       equation
         // add replacement for each derivative
         (varlst,ht) = makeAllDummyVarandDummyDerivativeRepl1(1,1,name,name,var,vars,so,varlst,ht);