[1.8>1.9] [MERGE #4650 @MikeHolman] fix issue where memop may scan fo…

…r missing values even if not in head segment

Merge pull request #4650 from MikeHolman:missingmemop

This may cause us to incorrectly set `HasNoMissingValues(false);`

OS: 15399206

lib/Runtime/Library/JavascriptArray.inl

@@ -989,7 +989,7 @@ SECOND_PASS:
                         CopyArray(current->elements + endIndex + 1, endSeg->length,
                             ((Js::SparseArraySegment<T>*)endSeg)->elements, endSeg->length);
                         LinkSegments((Js::SparseArraySegment<T>*)startPrev, current);
-                        if (HasNoMissingValues())
+                        if (current == head && HasNoMissingValues())
                         {
                             if (ScanForMissingValues<T>(endIndex + 1, endIndex + growby))
                             {