Add the WHERE_IN_EARLYOUT flag and use it to clarify the logic of this

optimization.

FossilOrigin-Name: 522f1eacc20f11002cad58232a7c2610f369568653510e54f46088f579f778dc

src/whereInt.h

@@ -557,3 +557,4 @@ void sqlite3WhereTabFuncArgs(Parse*, struct SrcList_item*, WhereClause*);
 #define WHERE_SKIPSCAN     0x00008000  /* Uses the skip-scan algorithm */
 #define WHERE_UNQ_WANTED   0x00010000  /* WHERE_ONEROW would have been helpful*/
 #define WHERE_PARTIALIDX   0x00020000  /* The automatic index is partial */
+#define WHERE_IN_EARLYOUT  0x00040000  /* Perhaps quit IN loops early */