7085 add support for "if" and "else" statements in dtrace Reviewed by: Adam Leventhal <ahl@delphix.com> Reviewed by: Sebastien Roy <sebastien.roy@delphix.com> Reviewed by: Paul Dagnelie <pcd@delphix.com> Reviewed by: Bryan Cantrill <bryan@joyent.com> Approved by: Richard Lowe <richlowe@richlowe.net>