[mypyc] Mark magic-overlapping error handler branches as rare

[JukkaL]

This might slightly improve performance when using native ints,
and it's consistent with how other error handler branches are
treated as rare.

Now the generated error handling code when calling a function
that returns a native int looks something like this (the
unlikely(...) part is new):

    cpy_r_r0 = CPyDef_f();
    cpy_r_r1 = cpy_r_r0 == -113;
    if (unlikely(cpy_r_r1)) goto CPyL2;