Suggestion: short-circuit type inference for ternary operators when both branches have the same type

From https://github.com/squidfunk/typescript-issue-38339-repro (courtesy of @squidfunk) **TypeScript Version:** 3.9.1-rc **Search Terms:** Subtype reduction, ternary **Code** ```ts import { JSX } from "preact" type IntrinsicElements = JSX.IntrinsicElements[keyof JSX.IntrinsicElements] export type IntrinsicElements2< T = IntrinsicElements > = { [K in keyof T]: K extends "accept" ? T[K] | [string] : T[K] } export type IntrinsicElements3< T = IntrinsicElements2 > = { [K in keyof T]: K extends "class" ? T[K] | [boolean] : T[K] } export type IntrinsicElements4< T = IntrinsicElements3 > = { [K in keyof T]: K extends "id" ? T[K] | [number] : T[K] } export function foo(): IntrinsicElements4 | undefined { let t: IntrinsicElements4 | undefined // this takes 2 seconds to check return t // this takes 10 seconds to check // return true ? t : t } ``` **Expected behavior:** `return t` and `return true ? t : t` check in a comparable amount of time **Actual behavior:** the ternary is much slower **Playground Link:** https://www.typescriptlang.org/play/?ts=Nightly#code/JYWwDg9gTgLgBAbzgKQMoA04F84DMoQhwBEYUApgIYDGMxAUPTAJ5jlwCSAdjFMFwGdg1AKIAbciHI8BcALwoMAOm69+Q0RKkyA2gGtyzCLkXoVPPoOHjJ0mAIC6jcgA9IsOCzacL661rsBACYAHno4OAAVeR81K01bGXoAPhiEcLgdAGk4fjgDIxNIhwAuOBzXGGkAE1liGmpyMDoMiIB+KOyHOAAfTIE4gHMnCIiyyK76LGc3aHgvdlVLDRttewBmMIjohSW-BLXglLSM7NyufMNjKNLyuEqauuoxSgEBBlG4Doms7r6dABGEAgEkoXBGo3Gk2m9Fc7nmrEWvniq0CABYtlEYnsUQEZOtjgp0hEznkCtdimUKi4qlxaiRgNUPqNvl1epkuABXEAA8hQCFjTq-KYzeF4TlcWjACAXXDAgAUAEoyjiVnj7Gj2RLquRcPxyNVEBkJPMVci1YkNVq6br9dUMgB6B0UGCcqAXeBOuAAMVeMAyLrdHqgnPYHXgZU9DrgqDEEAA7iKgA **Related Issues:** #38339