This makes the code slightly easier to follow, as the initialization
relies on an offset that overlapped with an equivalent set of macros
defined, which are used in other places already.

Author: Japin Li
Discussion: https://postgr.es/m/MEYP282MB1669FB410006758402F2C3A2B6E00@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM