Move the talloc_parent lookup down in a few hot paths.

talloc_parent is still 80% of our runtime, but likely talloc_parent
lookups will be reduced as we improve the handling of memory
ownership.

ir_dead_code.cpp

@@ -77,7 +77,6 @@ public:
 variable_entry *
 ir_dead_code_visitor::get_variable_entry(ir_variable *var)
 {
-   void *ctx = talloc_parent(var);
    assert(var);
    foreach_iter(exec_list_iterator, iter, this->variable_list) {
       variable_entry *entry = (variable_entry *)iter.get();
@@ -85,6 +84,8 @@ ir_dead_code_visitor::get_variable_entry(ir_variable *var)
 	 return entry;
    }
 
+   void *ctx = talloc_parent(var);
+
    variable_entry *entry = new(ctx) variable_entry(var);
    this->variable_list.push_tail(entry);
    return entry;