The currently implementation is somewhat minimal and only works in a very limited set of circumstances:
* the ref local is single-assignment
* the ref local is initialized directly with 'ldloca target; stloc ref_local',
not a derived pointer (e.g. 'ldloca target; ldflda F; stloc ref_local').
* all uses of the ref_local are immediately consuming the address
This improves variable splitting cases where the compiler re-uses a stack slot containing a reference,
e.g. in some cases of '?.' on Nullable<T>.