liveness: Move body_owner field from IrMaps to Liveness
liveness: Move body_owner field from IrMaps to Liveness

The Liveness struct is the only user of body_owner field.  Move the
field there.

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust