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_metadata/src/rmeta/encoder.rs

@@ -1804,7 +1804,7 @@ impl<'a, 'tcx> EncodeContext<'a, 'tcx> {
                     let simplified_self_ty = fast_reject::simplify_type(
                         self.tcx,
                         trait_ref.self_ty(),
-                        TreatParams::AsPlaceholders,
+                        TreatParams::AsInfer,
                     );
 
                     fx_hash_map