cmd/internal/gc: separate Name-only Node fields

Name will be converted from an anonymous to a
named field in a subsequent, automated CL.

No functional changes. Passes toolstash -cmp.

This reduces the size of gc.Node from 424 to 400 bytes.
This in turn reduces the permanent (pprof -inuse_space)
memory usage while compiling the test/rotate?.go tests:

test	old(MB)	new(MB)	change
rotate0	379.49	367.30	-3.21%
rotate1	373.42	361.59	-3.16%
rotate2	381.17	368.77	-3.25%
rotate3	374.30	362.48	-3.15%

Updates #9933.

Change-Id: I21479527c136add4f1efb9342774e3be3e276e83
Reviewed-on: https://go-review.googlesource.com/10120
Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/internal/gc/subr.go

@@ -380,6 +380,8 @@ func Nod(op int, nleft *Node, nright *Node) *Node {
 	switch op {
 	case OCLOSURE, ODCLFUNC:
 		n.Func = new(Func)
+	case ONAME:
+		n.Name = new(Name)
 	}
 	return n
 }
@@ -771,7 +773,6 @@ func treecopy(n *Node) *Node {
 		}
 		fallthrough
 
-		// fall through
 	case ONAME, OLITERAL, OTYPE:
 		m = n
 	}