Properly recast nested recursion pointers to their expanded layouts as needed

When we load a structure with a nested recursive pointer, the pointer
may be loaded opaquely. We need to refine it to the concrete type wanted
by the layout.

This conversion can be removed after we upgrade to LLVM 15.

Closes #5162

crates/compiler/test_gen/src/gen_tags.rs

@@ -2235,3 +2235,29 @@ fn refcount_nullable_unwrapped_needing_no_refcount_issue_5027() {
         RocStr
     );
 }
+
+#[test]
+#[cfg(any(feature = "gen-llvm"))]
+fn issue_5162_recast_nested_nullable_unwrapped_layout() {
+    assert_evals_to!(
+        indoc!(
+            r###"
+            app "test" provides [main] to "./platform"
+
+            Concept : [
+                AtomicConcept,
+                ExistentialRestriction { role : Str, concept : Concept }
+            ]
+
+            bottom : Concept
+            bottom = AtomicConcept
+
+            main =
+                when Dict.single bottom 0 is
+                    _ -> Bool.true
+            "###
+        ),
+        true,
+        bool
+    );
+}