IMPALA-856:NULLIFZERO and ZEROIFNULL

Both nullifzero and zeroifnull apply only to numeric types.
nullifzero(arg) - Return NULL if arg == 0, arg otherwise
zeroifnull(arg) - Return 0 if arg is NULL, arg otherwise

Change-Id: I41260de1edca2f9fcf50594fe137ca1f68f76056
Reviewed-on: http://gerrit.ent.cloudera.com:8080/1805
Reviewed-by: Srinath Shankar <sshankar@cloudera.com>
Tested-by: jenkins