Initial bits of constant expression evaluator

Currently only works for constants.  The rest will be added later.
diff --git a/ir.h b/ir.h
index 83e4f95..aa3e03a 100644
--- a/ir.h
+++ b/ir.h
@@ -40,6 +40,7 @@
 public:
    const struct glsl_type *type;
 
+   class ir_constant *constant_expression_value();
    virtual void accept(ir_visitor *) = 0;
 
    /**