glsl2: Use a better talloc context for ir_expression_flattening.

The instruction can be hung off of any other in the tree, even if the
other one will be deleted, since it'll get stolen to the shader's
context later if it's still live.

src/glsl/ir_expression_flattening.cpp

@@ -82,7 +82,7 @@ do_expression_flattening(exec_list *instructions,
 ir_rvalue *
 ir_expression_flattening_visitor::operand_to_temp(ir_rvalue *ir)
 {
-   void *ctx = talloc_parent(base_ir);
+   void *ctx = base_ir;
    ir_variable *var;
    ir_assignment *assign;