cmd/compile/internal: named types for Etype and Op in struct Node

Type Op is enfored now.
Type EType will need further CLs.
Added TODOs where Node.EType is used as a union type.
The TODOs have the format `TODO(marvin): Fix Node.EType union type.`.

Furthermore:
-The flag of Econv function in fmt.go is removed, since unused.
-Some cleaning along the way, e.g. declare vars first when getting initialized.

Passes go build -toolexec 'toolstash -cmp' -a std.

Fixes #11846

Change-Id: I908b955d5a78a195604970983fb9194bd9e9260b
Reviewed-on: https://go-review.googlesource.com/14956
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Marvin Stenger <marvin.stenger94@gmail.com>

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

@@ -509,7 +509,8 @@ func orderstmt(n *Node, order *Order) {
 			tmp1.Etype = 0 // now an rvalue not an lvalue
 		}
 		tmp1 = ordercopyexpr(tmp1, n.Left.Type, order, 0)
-		n.Right = Nod(int(n.Etype), tmp1, n.Right)
+		// TODO(marvin): Fix Node.EType type union.
+		n.Right = Nod(Op(n.Etype), tmp1, n.Right)
 		typecheck(&n.Right, Erv)
 		orderexpr(&n.Right, order, nil)
 		n.Etype = 0