Follow up on !4450 on static_cast<float>

As discussed at !4450 (comment 9755361)

In that case, there is a proper fix (add an overload of isnan to SIMDWrapper::scalar::float_v) but for now, the static_cast<float> is sufficient -- in the case where it won't work, it will refuse to compile (as opposed to the C-style cast, which would compile and invoke Undefined Behavior), and in the case it will compile, it will do what is expected.

But there should be a follow up MR which does this properly, for any reasonable float_v.

@wmorren @wouter @graven