pgsql: Properly fix AVX-512 CRC calculation bug

Properly fix AVX-512 CRC calculation bug

The problem that led to the workaround in f83f14881c7 was not in fact
a compiler bug, but a failure to zero the upper bits of the vector
register containing the initial scalar CRC value. Fix that and revert
the workaround.

Diagnosed-by: Nathan Bossart <nathandbossart@gmail.com>
Diagnosed-by: Raghuveer Devulapalli <raghuveer.devulapalli@intel.com>
Tested-by: Andy Fan <zhihuifan1213@163.com>
Tested-by: Soumyadeep Chakraborty <soumyadeep2007@gmail.com>
Reviewed-by: Nathan Bossart <nathandbossart@gmail.com>
Reviewed-by: Raghuveer Devulapalli <raghuveer.devulapalli@intel.com>
Discussion: https://postgr.es/m/PH8PR11MB82866B07AA6758D12F699C00FB70A@PH8PR11MB8286.namprd11.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/43da394304fba820830da2cef2c0214fe292c037

Modified Files
--------------
src/port/pg_crc32c_sse42.c        | 2 +-
src/port/pg_crc32c_sse42_choose.c | 4 +---
2 files changed, 2 insertions(+), 4 deletions(-)