casting operand to proper type in BlockIdGetBlockNumber

[Zhihong Yu]

Hi,
In test output, I saw:

src/backend/utils/adt/tid.c:112:16: runtime error: left shift of 65535 by
16 places cannot be represented in type 'int'

I think this was due to the left shift in BlockIdGetBlockNumber not
properly casting its operand.

Please see the proposed change in patch.

Thanks

get-block-number.patch
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