Cache the regularized form of union types (#37749)

* Cache the regularized form of union types

* Inline function because why not

* Introduce two fastpasths into isRelatedTo

src/compiler/types.ts

@@ -4653,6 +4653,8 @@ namespace ts {
     export interface UnionType extends UnionOrIntersectionType {
         /* @internal */
         resolvedReducedType: Type;
+        /* @internal */
+        regularType: UnionType;
     }
 
     export interface IntersectionType extends UnionOrIntersectionType {