glsl2: Move constant_expression_value method to ir_rvalue.

This prevents top-level callers from asking for the value of something
that is guaranteed not to have one.

src/glsl/ir_constant_expression.cpp

@@ -96,7 +96,7 @@ public:
 
 
 ir_constant *
-ir_instruction::constant_expression_value()
+ir_rvalue::constant_expression_value()
 {
    ir_constant_visitor visitor;