Fix nasty Simplifier scoping bug
This bug was somehow tickled by the new code for desugaring polymorphic bindings, but the bug has been there a long time. The bindings floated out in simplLazyBind, generated by abstractFloats, were getting processed by postInlineUnconditionally. But that was wrong because part of their scope has already been processed. That led to a bit of refactoring in the simplifier. See comments with Simplify.addPolyBind. In principle this might happen in 6.8.3, but in practice it doesn't seem to, so probably not worth merging.