Minor fix for typecast linter rule

Summary: intval($x, $y) cannot be safely changed to (int)$x if $y != 10.

Test Plan: Added test cases.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D14611