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>

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

@@ -32,7 +32,6 @@ type Node struct {
 	Name  *Name
 	Pack  *Node // real package for import . names
 	Curfn *Node // function for local variables
-	Alloc *Node // allocation call
 	Param *Param
 
 	// OPACK