Use != Positive rather than == Negative

Feels more complete, and for ImplPolarity has the side-effect of making
sure we also handle reservation impls correctly

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -710,7 +710,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // Don't register additional associated type bounds for negative bounds,
             // since we should have emitten an error for them earlier, and they will
             // not be well-formed!
-            if polarity == ty::PredicatePolarity::Negative {
+            if polarity != ty::PredicatePolarity::Positive {
                 assert!(
                     self.tcx().dcx().has_errors().is_some(),
                     "negative trait bounds should not have bindings",