cmd/compile: remove Node.Alloc

$ sizeof -p cmd/compile/internal/gc Node
Node 240
$

Change-Id: Id12710c480ed4e0a5bf4f5006f6bd56ef91a2af1
Reviewed-on: https://go-review.googlesource.com/10525
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
diff --git a/src/cmd/compile/internal/gc/range.go b/src/cmd/compile/internal/gc/range.go
index 8111f8a..26f05d9 100644
--- a/src/cmd/compile/internal/gc/range.go
+++ b/src/cmd/compile/internal/gc/range.go
@@ -303,7 +303,7 @@
 		ha := a
 
 		th := hiter(t)
-		hit := n.Alloc
+		hit := prealloc[n]
 		hit.Type = th
 		n.Left = nil
 		keyname := newname(th.Type.Sym)      // depends on layout of iterator struct.  See reflect.go:hiter