Shrink hir::def::Res.

`Res::SelfTy` currently has two `Option`s. When the second one is `Some`
the first one is never consulted. So we can split it into two variants,
`Res::SelfTyParam` and `Res::SelfTyAlias`, reducing the size of `Res`
from 24 bytes to 12. This then shrinks `hir::Path` and
`hir::PathSegment`, which are the HIR types that take up the most space.

compiler/rustc_infer/src/infer/error_reporting/need_type_info.rs

@@ -1021,7 +1021,7 @@ impl<'a, 'tcx> FindInferSourceVisitor<'a, 'tcx> {
                                     }
                                     // There cannot be inference variables in the self type,
                                     // so there's nothing for us to do here.
-                                    Res::SelfTy { .. } => {}
+                                    Res::SelfTyParam { .. } | Res::SelfTyAlias { .. } => {}
                                     _ => warn!(
                                         "unexpected path: def={:?} substs={:?} path={:?}",
                                         def, substs, path,

compiler/rustc_infer/src/infer/error_reporting/nice_region_error/trait_impl_difference.rs

@@ -156,7 +156,8 @@ impl<'tcx> Visitor<'tcx> for TypeParamSpanVisitor<'tcx> {
                 [segment]
                     if matches!(
                         segment.res,
-                        Res::SelfTy { trait_: _, alias_to: _ }
+                        Res::SelfTyParam { .. }
+                            | Res::SelfTyAlias { .. }
                             | Res::Def(hir::def::DefKind::TyParam, _)
                     ) =>
                 {