(US) code requires a minimum of 15" on either side of the toilet centerline. If you still have at least that after adding insulation, I wouldn't worry about it. The alternative is to break up the concrete and move it where you want. This can be expensive or labor intensive if you do it yourself. If this puts the toilet closer to a 10" rough-in verses a 12", it would be less expensive to replace the toilet with a 10" rough-in model.