Generalise the existing SIGN_EXTEND to SIGN_EXTEND_VECTOR_INREG combine to support zero extension as well and get rid of a lot of unnecessary ANY_EXTEND + mask patterns.
This won't solve the issues with PR25718 (load+zext 8xi8 to 8xi32) but is one of several things that needs to be done at the same time.
Igor/Elena - can you advise why the masks aren't being folded into the VPMOVZX instructions on skylake targets any more please?