SharedUnionFieldsDeep: Skip if input type is not a union type.

[Emiyaaaaa]

I found that SharedUnionFieldsDeep have some isuue on handling some variable-length array like

type Arr = [...number[], string]

type R = SharedUnionFieldsDeep<{a: Arr} | {a: Arr}>
//=> {a: (number | string)[]}

I haven't found a good solution currently, but I can skip the non-union type(same type) to alleviate it now.

and this PR will also improves the performance of SharedUnionFieldsDeep