Fix failure of AVX2-accelerated mb_check_encoding on 32-bit MS Windows

Thanks to Ilija Tovilo for noticing and reporting this problem. Thanks
also to Michael Voříšek for finding the StackOverflow post which
explained the reason for the failure.

Author: alexdowad

ext/mbstring/mbstring.c

@@ -5155,7 +5155,12 @@ static bool mb_fast_check_utf8_avx2(zend_string *str)
 			goto check_operand;
 		case 7:
 		case 8:
-			operand = _mm256_set_epi64x(0, 0, 0, *((int64_t*)p));
+			/* This was originally: operand = _mm256_set_epi64x(0, 0, 0, *((int64_t*)p));
+			 * However, that caused test failures on 32-bit MS Windows
+			 * (Bad 7/8-byte UTF-8 strings would be wrongly passed through as 'valid')
+			 * It seems this is caused by a bug in MS Visual C++
+			 * Ref: https://stackoverflow.com/questions/37509129/potential-bug-in-visual-studio-c-compiler-or-in-intel-intrinsics-avx2-mm256-s */
+			operand = _mm256_set_epi32(0, 0, 0, 0, 0, 0, ((int32_t*)p)[1], ((int32_t*)p)[0]);
 			goto check_operand;
 		case 9:
 			operand = _mm256_set_m128i(_mm_setzero_si128(), _mm_srli_si128(_mm_loadu_si128((__m128i*)(p - 6)), 6));

ext/mbstring/tests/utf_encodings.phpt

@@ -5,7 +5,6 @@ mbstring
 --SKIPIF--
 <?php
 if (getenv("SKIP_SLOW_TESTS")) die("skip slow test");
-if (substr(PHP_OS, 0, 3) === 'WIN' && PHP_INT_SIZE === 4) die("skip not for Windows x86");
 ?>
 --FILE--
 <?php