Fix broken constructor of ir_instruction base class

Make the constructor inline-able, and don't try to initialize it as a
simple_node.  It hasn't been derived from simple_node in a long time.

ir.cpp

@@ -26,13 +26,6 @@
 #include "ir.h"
 #include "glsl_types.h"
 
-ir_instruction::ir_instruction(int mode)
-{
-   this->mode = mode;
-   make_empty_list(this);
-}
-
-
 ir_assignment::ir_assignment(ir_instruction *lhs, ir_instruction *rhs,
 			     ir_expression *condition)
    : ir_instruction(ir_op_assign)

ir.h

@@ -53,7 +53,11 @@ public:
    virtual void accept(ir_visitor *) = 0;
 
 protected:
-   ir_instruction(int mode);
+   ir_instruction(int mode)
+      : mode(mode)
+   {
+      /* empty */
+   }
 
 private:
    /**