added primitive for dot products xx, yy, and xy at the same time
added primitive for dot products xx, yy, and xy at the same time

spotify/annoy,eddelbuettel/annoy,eddelbuettel/annoy,tjrileywisc/annoy,tjrileywisc/annoy,eddelbuettel/annoy,tjrileywisc/annoy,spotify/annoy,spotify/annoy,spotify/annoy,eddelbuettel/annoy,tjrileywisc/annoy