[bugfix] Explicitly cast argument types for Simd::cond

`Simd::cond(a,b,c)` only has overloads for the cases where
`b` and `c` have the same type. If this is not the case
(which seems to happen for certain distributions and compilers)
the compiler cannot deduce the template parameter for the type.

It turns out that in all cases where this happens, the first
argument is constructed by explicitly casting to a type. In
all these cases the present patch adds a cast to this type
for the second argument, too.

WARNING: I did not look at what the actual code does. However,
if the implementation of `Simd::cond()` makes and the existing
cast make any sense, that's the only possible type we can commit to.