Regression fix for the "reverse" addition

Introduced by a99f132f686c19499e06217d8ad3d9bfec9c6560.

`self.view.sel()` in fact returns an interator (or, "RegionSet"s behave like
iterators) which would adapt to changes from `view.replace()`. However, if we
construct the region list before making the changes this will be skipped.