pgsql: Optimize visibilitymap_count() with AVX-512 instructions.

Optimize visibilitymap_count() with AVX-512 instructions.

Commit 792752af4e added infrastructure for using AVX-512 intrinsic
functions, and this commit uses that infrastructure to optimize
visibilitymap_count(). Specificially, a new pg_popcount_masked()
function is introduced that applies a bitmask to every byte in the
buffer prior to calculating the population count, which is used to
filter out the all-visible or all-frozen bits as needed. Platforms
without AVX-512 support should also see a nice speedup due to the
reduced number of calls to a function pointer.

Co-authored-by: Ants Aasma
Discussion: https://postgr.es/m/BL1PR11MB5304097DF7EA81D04C33F3D1DCA6A%40BL1PR11MB5304.namprd11.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/41c51f0c68b21b4603bd2a9c3d3ad017fdd22627

Modified Files
--------------
src/backend/access/heap/visibilitymap.c | 25 ++-----
src/include/port/pg_bitutils.h | 34 +++++++++
src/port/pg_bitutils.c | 126 ++++++++++++++++++++++++++++++++
src/port/pg_popcount_avx512.c | 60 +++++++++++++++
4 files changed, 225 insertions(+), 20 deletions(-)