Function in_vector_file 

pub fn in_vector_file(ty: Type) -> bool

Whether a value of that type lives in a vector register, which is the two formats the machine computes in plus the one it only moves.

The question a register file is picked by, asked here so that the three places that pick one are reading the same answer. A value put in the wrong file is a value every instruction that then touches it is the wrong instruction for.