That's worth it, as fmgr_builtins is frequently accessed, and as
fmgr_builtins is one of the biggest constant variables in a backend.

On most 64bit systems this will change the size of the struct from
32byte to 24bytes. While that could make indexing into the array
marginally more expensive, the higher cache hit ratio is worth more,
especially because these days fmgr_builtins isn't searched with a
binary search anymore (c.f. 212e6f34).

Discussion: https://postgr.es/m/20181016201145.aa2dfeq54rhqzron@alap3.anarazel.de