Fix bug in CEGIS

src/main/scala/leon/purescala/TreeOps.scala

src/main/scala/leon/refactor/Repairman.scala

src/main/scala/leon/synthesis/CostModel.scala

src/main/scala/leon/synthesis/rules/Cegis.scala

src/main/scala/leon/synthesis/rules/GuidedDecomp.scala

src/main/scala/leon/utils/Simplifiers.scala