Change LIKELY() and UNLIKELY() to cast their first argument to a bool.

This avoids problems such as pointers being compared with the literal value 1.
Suggested by Zefram.

Note that this is technically a change of behaviour, as the macro EXPECT(),
which they are both both wrappers for, returns its value, so they will now
be returning TRUE or FALSE, rather than the actual value passed to them.
However, all uses in the core and on CPAN are only within if() statements, so
this should not make any difference in practice.