Properly fix AVX-512 CRC calculation bug

The problem that led to the workaround in f83f14881c 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

src/port/pg_crc32c_sse42.c

@@ -123,7 +123,7 @@ pg_comp_crc32c_avx512(pg_crc32c crc, const void *data, size_t len)
 		__m512i		k;
 
 		k = _mm512_broadcast_i32x4(_mm_setr_epi32(0x740eef02, 0, 0x9e4addf8, 0));
-		x0 = _mm512_xor_si512(_mm512_castsi128_si512(_mm_cvtsi32_si128(crc0)), x0);
+		x0 = _mm512_xor_si512(_mm512_zextsi128_si512(_mm_cvtsi32_si128(crc0)), x0);
 		buf += 64;
 
 		/* Main loop. */

src/port/pg_crc32c_sse42_choose.c

@@ -95,9 +95,7 @@ pg_comp_crc32c_choose(pg_crc32c crc, const void *data, size_t len)
 			__cpuidex(exx, 7, 0);
 #endif
 
-#if defined(__clang__) && !defined(__OPTIMIZE__)
-			/* Some versions of clang are broken at -O0 */
-#elif defined(USE_AVX512_CRC32C_WITH_RUNTIME_CHECK)
+#ifdef USE_AVX512_CRC32C_WITH_RUNTIME_CHECK
 			if (exx[2] & (1 << 10) &&	/* VPCLMULQDQ */
 				exx[1] & (1 << 31)) /* AVX512-VL */
 				pg_comp_crc32c = pg_comp_crc32c_avx512;