We have VSX store instructions that will store a single element from a vector without modifying it in any way. Previous generation cores can do this for word-sized elements and Power9 can also do it for half-word and byte-sized elements.
The TableGen patterns for the word-sized versions were missing - this patch adds them.
Furthermore, it provides the information about the cost of such a combine - zero cost when the index is the element number that the instruction stores, cost of 3 for other elements. The cost for the other elements is because a vector permute is needed to shift the element and it's only worthwhile if keeping the value as a vector reduces the cost enough to offset the cost of the permute.