cmd/compile/internal/ir: remove OrigNode

The OrigNode functionality used to be relevant to the typecheck
frontend, because we wanted to report errors using the same syntax as
the user originally wrote. However, now that types2 handles all
spec-required error diagnostics, there's no need to preserve original
nodes anymore.

Change-Id: I64a0540b8952513913021e7b84d165beb1f9f801
Reviewed-on: https://go-review.googlesource.com/c/go/+/526397
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

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

@@ -188,7 +188,6 @@ func (n *BinaryExpr) SetOp(op Op) {
 // A CallExpr is a function call X(Args).
 type CallExpr struct {
 	miniExpr
-	origNode
 	X         Node
 	Args      Nodes
 	RType     Node    `mknode:"-"` // see reflectdata/helpers.go
@@ -200,7 +199,6 @@ type CallExpr struct {
 func NewCallExpr(pos src.XPos, op Op, fun Node, args []Node) *CallExpr {
 	n := &CallExpr{X: fun}
 	n.pos = pos
-	n.orig = n
 	n.SetOp(op)
 	n.Args = args
 	return n
@@ -234,7 +232,6 @@ type ClosureExpr struct {
 // Before type-checking, the type is Ntype.
 type CompLitExpr struct {
 	miniExpr
-	origNode
 	List     Nodes // initialized values
 	RType    Node  `mknode:"-"` // *runtime._type for OMAPLIT map types
 	Prealloc *Name
@@ -251,7 +248,6 @@ func NewCompLitExpr(pos src.XPos, op Op, typ *types.Type, list []Node) *CompLitE
 	if typ != nil {
 		n.SetType(typ)
 	}
-	n.orig = n
 	return n
 }