Use __builtin_clz() where available. Where it isn't, we can still win
a little by using the pg_leftmost_one_pos[] lookup table instead of
having a private table.
Also drop the initial right shift by ALLOC_MINBITS in favor of
subtracting ALLOC_MINBITS from the leftmost-one-pos result. This
is a win because the compiler can fold that adjustment into other
constants it'd have to add anyway, making the shift-removal free.
Also, we can explain this coding as an unrolled form of
pg_leftmost_one_pos32(), even though that's a bit ahistorical
since it long predates pg_bitutils.h.
John Naylor, with some cosmetic adjustments by me
Discussion: https://postgr.es/m/CACPNZCuNUGMxjK7WTn_=WZnRbfASDdBxmjsVf2+m9MdmeNw_sg@mail.gmail.com
0ce38730ac