pgsql: Micro-optimize AllocSetFreeIndex() by reference to pg_bitutils c

Micro-optimize AllocSetFreeIndex() by reference to pg_bitutils code.

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0ce38730ac72029f3f2c95ae80b44f5b9060cbcc

Modified Files
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src/backend/utils/mmgr/aset.c | 56 ++++++++++++++++++++++++-------------------
1 file changed, 31 insertions(+), 25 deletions(-)