Fix unwanted de-Boolification of 1 arg to nqp::if/unless
Fix unwanted de-Boolification of 1 arg to nqp::if/unless

Only perform the optimization for if/unless that have the `else`
clause that would be returned. Otherwise, we can end up returning
the de-boolified Int instead.

Bug find: https://irclog.perlgeek.de/perl6-dev/2017-07-10#i_14852407

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