[dev.regabi] cmd/compile: simplify export data representation of nil

The handling of ONIL and Orig has been a mess for a while, and dates
back to how fmt.go used to print out typed nils. That hasn't applied
for a while, but we've kept dragging it along to appease toolstash
with the intention of someday finally removing it.

Today is that day.

Change-Id: I9a441628e53068ab1993cd2b67b977574d8117b7
Reviewed-on: https://go-review.googlesource.com/c/go/+/274212
Trust: Matthew Dempsky <mdempsky@google.com>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Russ Cox <rsc@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>

src/cmd/compile/internal/gc/iexport.go

@@ -1201,11 +1201,7 @@ func (w *exportWriter) expr(n ir.Node) {
 		if !n.Type().HasNil() {
 			base.Fatalf("unexpected type for nil: %v", n.Type())
 		}
-		if orig := ir.Orig(n); orig != nil && orig != n {
-			w.expr(orig)
-			break
-		}
-		w.op(ir.OLITERAL)
+		w.op(ir.ONIL)
 		w.pos(n.Pos())
 		w.typ(n.Type())
 

src/cmd/compile/internal/gc/iimport.go

@@ -809,20 +809,19 @@ func (r *importReader) node() ir.Node {
 	// case OPAREN:
 	// 	unreachable - unpacked by exporter
 
-	// case ONIL:
-	//	unreachable - mapped to OLITERAL
+	case ir.ONIL:
+		pos := r.pos()
+		typ := r.typ()
+
+		n := npos(pos, nodnil())
+		n.SetType(typ)
+		return n
 
 	case ir.OLITERAL:
 		pos := r.pos()
 		typ := r.typ()
 
-		var n ir.Node
-		if typ.HasNil() {
-			n = nodnil()
-		} else {
-			n = ir.NewLiteral(r.value(typ))
-		}
-		n = npos(pos, n)
+		n := npos(pos, ir.NewLiteral(r.value(typ)))
 		n.SetType(typ)
 		return n