fix for ticket:5054

- proper prefixing (subscript transfer) of condition in Equation.IF

Belonging to [master]:
  - #2591
  - OpenModelica/OpenModelica-testsuite#1007

Author: adrpo

Compiler/NFFrontEnd/NFFlatten.mo

@@ -778,6 +778,9 @@ algorithm
     bl := match b
       case Equation.Branch.BRANCH(cond, var, eql)
         algorithm
+          // flatten the condition first
+          cond := flattenExp(cond, prefix);
+          // flatten the equations
           eql := flattenEquations(eql, prefix);
 
           if Expression.isTrue(cond) and listEmpty(bl) then