Reorder hir::Expr fields.

On 64-bit platforms this reduces the size of `Expr` from 96 bytes to 88
bytes.

src/librustc/hir/mod.rs

@@ -840,8 +840,8 @@ pub enum UnsafeSource {
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash)]
 pub struct Expr {
     pub id: NodeId,
-    pub node: Expr_,
     pub span: Span,
+    pub node: Expr_,
     pub attrs: ThinVec<Attribute>,
 }