Auto merge of #97024 - lcnr:simplify_type-sus, r=<try>

`simplify_type` improvements and cursed docs

the existing `TreatParams` enum pretty much mixes everything up. Not sure why this looked right to me in #94057

This also includes two changes which impact perf:
- `ty::Projection` with inference vars shouldn't be treated as a rigid type, even if fully normalized
- `ty::Placeholder` only unifies with itself, so actually return `Some` for them

r? `@nikomatsakis`

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2148,13 +2148,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                         let simplified_obligation_ty = fast_reject::simplify_type(
                             self.tcx(),
                             obligation_ty,
-                            TreatParams::AsBoundTypes,
-                        );
-                        let simplified_impl_ty = fast_reject::simplify_type(
-                            self.tcx(),
-                            impl_ty,
-                            TreatParams::AsPlaceholders,
+                            TreatParams::AsPlaceholder,
                         );
+                        let simplified_impl_ty =
+                            fast_reject::simplify_type(self.tcx(), impl_ty, TreatParams::AsInfer);
 
                         simplified_obligation_ty.is_some()
                             && simplified_impl_ty.is_some()