pgsql: Rename TRY_POPCNT_FAST to TRY_POPCNT_X86_64.

Rename TRY_POPCNT_FAST to TRY_POPCNT_X86_64.

This macro protects x86_64-specific code, and a subsequent commit
will introduce AArch64-specific versions of that code. To prevent
confusion, let's rename it to clearly indicate that it's for
x86_64. We should likely move this code to its own file (perhaps
merging it with the AVX-512 popcount code), but that is left as a
future exercise.

Reviewed-by: "Chiranmoy(dot)Bhattacharya(at)fujitsu(dot)com" <Chiranmoy(dot)Bhattacharya(at)fujitsu(dot)com>
Reviewed-by: John Naylor <johncnaylorls(at)gmail(dot)com>
Discussion: https://postgr.es/m/010101936e4aaa70-b474ab9e-b9ce-474d-a3ba-a3dc223d295c-000000%40us-west-2.amazonses.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9ac6f7e7ceb6e7c9b168bbf02bbe4662782dfa6e

Modified Files
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src/include/port/pg_bitutils.h | 6 +++---
src/port/pg_bitutils.c | 14 +++++++-------
src/port/pg_popcount_avx512.c | 8 ++++----
3 files changed, 14 insertions(+), 14 deletions(-)