Balance sidebar Deref cycle check with main content

The `Deref` cycle checks added as part of #80653 were "unbalanced" in the sense
that the main content code path checks for cycles _before_ descending, while the
sidebar checks _after_. Checking _before_ is correct, so this changes the
sidebar path to match the main content path.

src/test/rustdoc-ui/deref-generic.rs

@@ -0,0 +1,15 @@
+// check-pass
+// #81395: Fix ICE when recursing into Deref target only differing in type args
+
+pub struct Generic<T>(T);
+
+impl<'a> std::ops::Deref for Generic<&'a mut ()> {
+    type Target = Generic<&'a ()>;
+    fn deref(&self) -> &Self::Target {
+        unimplemented!()
+    }
+}
+
+impl<'a> Generic<&'a ()> {
+    pub fn some_method(&self) {}
+}