ir_constant_expression: Remove pointless use of variable_referenced.

ir_dereference_variable always references an ir_variable, so there's no
point in calling a function and NULL-checking the result.
diff --git a/src/glsl/ir_constant_expression.cpp b/src/glsl/ir_constant_expression.cpp
index 186d0c4..cb07f38 100644
--- a/src/glsl/ir_constant_expression.cpp
+++ b/src/glsl/ir_constant_expression.cpp
@@ -695,10 +695,7 @@
 ir_constant *
 ir_dereference_variable::constant_expression_value()
 {
-   ir_variable *var = this->variable_referenced();
-   if (var && var->constant_value)
-      return var->constant_value->clone(NULL);
-   return NULL;
+   return var->constant_value ? var->constant_value->clone(NULL) : NULL;
 }