[Eva] Float_interval: slightly rewrites backward_is_finite and backward_is_nan.

These backward propagators takes a boolean argument [positive] that indicates
whether the value to be reduced is finite (respectively NaN) or not.

[backward_is_nan] replaces the two previous functions [backward_is_nan] and
[backward_is_not_nan].

Implements the backward propagation [is_finite ~positive:false] that corresponds
to !\is_finite(f).