don't generate n-ary add instructions for switch's default case

Address issue #147, where Raimondas pointed out that we still
generated n-ary add instructions.
drop GEP for now, it's broken WRT the pass
Merge pull request #148 from chenyang78/n-ary

don't generate n-ary add instructions for switch's default case
Merge remote-tracking branch 'origin/master' into synthesis-pass-support