[dev.regabi] cmd/compile: remove use of Initorder, Offset Node fields for initorder
The initorder pass is already making heavy use of maps,
and it is concerned with relatively few nodes (only the assignments
in package-level variable declarations). The tracking of init order
for these nodes can be done with another map instead of storing
the bits directly in the Node representations.
This will let us drop Offset_ from AssignStmt and AssignListStmt
and drop Initorder from all nodes.