Put shift stacktrace creation behind a DEBUG flag

[nomisRev]

Shift doesn't need stack trace creation, and it's almost never useful for shift. So ideally we put this behind a DEBUG flag, similar to how KotlinX deals with this.

https://github.com/Kotlin/kotlinx.coroutines/blob/d88a8c227a295f91251960c8fe6b21a02ddf704b/kotlinx-coroutines-core/jvm/src/Debug.kt#L36