[cfe] Use identical in subtype check

This improves the subtype check by ~27%. Using subtype_measure.dart the
test result was: TTestResult[significant: -27.74% +/- 2.03% (-34846.20 +/- 2548.23) (at least -32297.97)]

Change-Id: I908fe8807f7b265c59f6e5f83e3e8a444eb11743
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/387965
Reviewed-by: Jens Johansen <jensj@google.com>
Commit-Queue: Johnni Winther <johnniwinther@google.com>

pkg/kernel/lib/src/ast/types.dart

@@ -525,7 +525,8 @@ sealed class DartType extends Node implements SharedType {
   R accept1<R, A>(DartTypeVisitor1<R, A> v, A arg);
 
   @override
-  bool operator ==(Object other) => equals(other, null);
+  bool operator ==(Object other) =>
+      identical(this, other) || equals(other, null);
 
   /// The nullability declared on the type.
   ///
@@ -1747,7 +1748,8 @@ class NamedType extends Node
   DartType get typeShared => type;
 
   @override
-  bool operator ==(Object other) => equals(other, null);
+  bool operator ==(Object other) =>
+      identical(this, other) || equals(other, null);
 
   bool equals(Object other, Assumptions? assumptions) {
     return other is NamedType &&
@@ -2237,9 +2239,6 @@ class StructuralParameterType extends DartType {
   @override
   void visitChildren(Visitor v) {}
 
-  @override
-  bool operator ==(Object other) => equals(other, null);
-
   @override
   bool equals(Object other, Assumptions? assumptions) {
     if (identical(this, other)) {

pkg/kernel/lib/src/types.dart

@@ -63,6 +63,7 @@ class Types with StandardBounds {
   }
 
   IsSubtypeOf performSubtypeCheck(DartType s, DartType t) {
+    if (s == t) return const IsSubtypeOf.success();
     switch ((s, t)) {
       // TODO(johnniwinther,cstefantsova): Ensure complete handling of
       // InvalidType in the subtype relation.