Rollup merge of #113698 - compiler-errors:rpitit-check, r=spastorino

Make it clearer that we're just checking for an RPITIT

Tiny nit to use `is_impl_trait_in_trait` more, to make it clearer that we're just checking whether a def-id is an RPITIT, rather than doing something meaningful with the `opt_rpitit_info`.

r? `@spastorino`

compiler/rustc_passes/src/dead.rs

@@ -276,7 +276,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             }
 
             // Avoid accessing the HIR for the synthesized associated type generated for RPITITs.
-            if self.tcx.opt_rpitit_info(id.to_def_id()).is_some() {
+            if self.tcx.is_impl_trait_in_trait(id.to_def_id()) {
                 self.live_symbols.insert(id);
                 continue;
             }