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_mir_build/src/thir/pattern/mod.rs

@@ -433,7 +433,8 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                 | DefKind::AssocTy,
                 _,
             )
-            | Res::SelfTy { .. }
+            | Res::SelfTyParam { .. }
+            | Res::SelfTyAlias { .. }
             | Res::SelfCtor(..) => PatKind::Leaf { subpatterns },
             _ => {
                 let pattern_error = match res {