[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.

Passes buildall w/ toolstash -cmp.

Change-Id: I151c64e84670292c2004da4e8e3d0660a88e3df3
Reviewed-on: https://go-review.googlesource.com/c/go/+/277917
Trust: Russ Cox <rsc@golang.org>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/ir/node.go

@@ -102,8 +102,6 @@ type Node interface {
 	SetBounded(x bool)
 	Typecheck() uint8
 	SetTypecheck(x uint8)
-	Initorder() uint8
-	SetInitorder(x uint8)
 	NonNil() bool
 	MarkNonNil()
 	HasCall() bool