Fix devirtualization across genericness in the hierarchy (dotnet#108442)

This code was trying to answer question: "Was this method overriden by something else in a more derived class"? It was walking the base hierarchy in canonical form, but that was leading to methods not resolving at all. The fix is to walk the non-canonical hierarchy and canonicalize after we resolved everything.

I ran into this in dotnet#108379 that unlocked more whole program devirtualization and `StringSearchValuesBase` is in this shape.

src/coreclr/tools/aot/ILCompiler.Compiler/Compiler/ILScanner.cs

@@ -559,7 +559,7 @@ static List<MethodDesc> BuildVTable(NodeFactory factory, TypeDesc currentType, T
                                 if (currentType == null)
                                     return vtable;
 
-                                BuildVTable(factory, currentType.BaseType?.ConvertToCanonForm(CanonicalFormKind.Specific), implType, vtable);
+                                BuildVTable(factory, currentType.BaseType, implType, vtable);
 
                                 IReadOnlyList<MethodDesc> slice = factory.VTable(currentType).Slots;
                                 foreach (MethodDesc decl in slice)
@@ -571,19 +571,19 @@ static List<MethodDesc> BuildVTable(NodeFactory factory, TypeDesc currentType, T
                                 return vtable;
                             }
 
-                            baseType = canonType.BaseType?.ConvertToCanonForm(CanonicalFormKind.Specific);
-                            if (!canonType.IsArray && baseType != null)
+                            baseType = type.BaseType;
+                            if (!type.IsArray && baseType != null)
                             {
                                 if (!vtables.TryGetValue(baseType, out List<MethodDesc> baseVtable))
                                     vtables.Add(baseType, baseVtable = BuildVTable(factory, baseType, baseType, new List<MethodDesc>()));
 
-                                if (!vtables.TryGetValue(canonType, out List<MethodDesc> vtable))
-                                    vtables.Add(canonType, vtable = BuildVTable(factory, canonType, canonType, new List<MethodDesc>()));
+                                if (!vtables.TryGetValue(type, out List<MethodDesc> vtable))
+                                    vtables.Add(type, vtable = BuildVTable(factory, type, type, new List<MethodDesc>()));
 
                                 for (int i = 0; i < baseVtable.Count; i++)
                                 {
                                     if (baseVtable[i] != vtable[i])
-                                        _overriddenMethods.Add(baseVtable[i]);
+                                        _overriddenMethods.Add(baseVtable[i].GetCanonMethodTarget(CanonicalFormKind.Specific));
                                 }
                             }
                         }

src/tests/nativeaot/SmokeTests/UnitTests/Devirtualization.cs

@@ -12,6 +12,7 @@ class Devirtualization
     internal static int Run()
     {
         RegressionBug73076.Run();
+        RegressionGenericHierarchy.Run();
         DevirtualizationCornerCaseTests.Run();
         DevirtualizeIntoUnallocatedGenericType.Run();
 
@@ -53,6 +54,36 @@ public static void Run()
         }
     }
 
+    class RegressionGenericHierarchy
+    {
+        class Base<T>
+        {
+            public virtual string Print() => "Base<T>";
+        }
+
+        class Mid : Base<Atom>
+        {
+            public override string Print() => "Mid";
+            public override string ToString() => Print();
+        }
+
+        class Derived : Mid
+        {
+            public override string Print() => "Derived";
+        }
+
+        class Atom { }
+
+        public static void Run()
+        {
+            if (Get().ToString() != "Derived")
+                throw new Exception();
+
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static object Get() => new Derived();
+        }
+    }
+
     class DevirtualizationCornerCaseTests
     {
         interface IIntf1