Avoid expanding equations

- In RemoveSimpleEquations, check if simplifying a vector equation after
  expanding it causes simplify to perform any changes. If no change
  occurs, it is unlinkely that removeSimpleEquations will work on the
  equation, so discard the expansion.

This fixes the performance problem reported in ticket:4453

Compiler/BackEnd/RemoveSimpleEquations.mo

@@ -537,7 +537,7 @@ algorithm
     simpleContainer := List.listArrayReverse(simpleContainerIn);
     // collect and handle sets
     (vars, eqnslst, shared, repl) := handleSets(arrayLength(simpleContainer), 1, simpleContainer, iMT, iUnreplaceable, iVars, iEqnslst, ishared, iRepl);
-      //BackendVarTransform.dumpReplacements(repl);
+    //BackendVarTransform.dumpReplacements(repl);
 
     // perform replacements and try again
     (eqnslst, b1) := BackendVarTransform.replaceEquations(eqnslst, repl, SOME(BackendVarTransform.skipPreChangeEdgeOperator));
@@ -1154,8 +1154,8 @@ algorithm
   ds := Expression.dimensionsSizes(dims);
   subslst := List.map(ds, Expression.dimensionSizeSubscripts);
   subslst := Expression.rangesToSubscripts(subslst);
-  elst1 := List.map1r(subslst, Expression.applyExpSubscripts, lhs);
-  elst2 := List.map1r(subslst, Expression.applyExpSubscripts, rhs);
+  (elst1,true) := List.mapFold(subslst, function Expression.applyExpSubscriptsFoldCheckSimplify(exp=lhs), false);
+  (elst2,true) := List.mapFold(subslst, function Expression.applyExpSubscriptsFoldCheckSimplify(exp=rhs), false) "Do not expand equation if it doesn't help with anything... Like x=f(...); => x[1]=f()[1], ..., x[n]=f()[n]";
   outTpl := List.threadFold2(elst1, elst2, simpleEquationAcausal, eqnAttributes, true, inTpl);
 end simpleArrayEquationAcausal;