[PATCH] Teach the DAGCombiner how to fold a OR of two shufflevector into a single shufflevector node

[Tim Northover]

> It is okay to generate new shuffles from a collection of insert/extracts
> because the new shuffle instructions can’t be worse then a collection
> of inserts and extracts.

Thanks Nadav; it sounds like I should remove the TODO and comment on
the scope of that function then as part of whatever I do.

Tim.