Add precondition back to firstNeqStride

ConflictSet.cpp

@@ -626,6 +626,7 @@ bool compareStride(const uint8_t *ap, const uint8_t *bp) {
   return eq;
 }
 
+// Precondition: ap[:kStride] != bp[:kStride]
 int firstNeqStride(const uint8_t *ap, const uint8_t *bp) {
 #if defined(HAS_AVX)
   static_assert(kStride == 64);