Move the "same instance" check at `mergeFrom(MessageLite)` into `GeneratedMessageLite`.

In `AbstractMessageLite.Builder.mergeFrom(MessageLite)`, it either goes into (full runtime) `AbstractMessage` which supports merge from a different instance but the same *descriptor* type, or into the lite runtime path which should not support it in the absence of descriptors .e.g in case of `DynamicMessage`.

PiperOrigin-RevId: 924941126