Merged ZSTD_preserveUnsortedMark() into ZSTD_reduceIndex()

as it's faster, due to one memory scan instead of two
(confirmed by microbenchmark).

Note : as ZSTD_reduceIndex() is rarely invoked,
it does not translate into a visible gain.
Consider it an exercise in auto-vectorization and micro-benchmarking.

lib/compress/zstd_compress_internal.h

@@ -32,6 +32,12 @@ extern "C" {
 ***************************************/
 static const U32 g_searchStrength = 8;
 #define HASH_READ_SIZE 8
+#define ZSTD_DUBT_UNSORTED_MARK 1   /* For btlazy2 strategy, index 1 now means "unsorted".
+                                       It could be confused for a real successor at index "1", if sorted as larger than its predecessor.
+                                       It's not a big deal though : candidate will just be sorted again.
+                                       Additionnally, candidate position 1 will be lost.
+                                       But candidate 1 cannot hide a large tree of candidates, so it's a minimal loss.
+                                       The benefit is that ZSTD_DUBT_UNSORTED_MARK cannot be misdhandled after table re-use with a different strategy */
 
 
 /*-*************************************