`bigfield`: Investigate inefficiencies in `conditional_negate` of bigfield

[suyash67]

An old TODO in bigfield's conditional_negate function indicates inefficiency in the way we implement this. This function is used in biggroup's conditional_negate function and few other places.

There could be two ways to optimise:

1. Use conditional_select function as a subroutine if conditional_select is a more a efficient method.
2. Optimise the existing logic.