glsl: Separate overlapping sentinel nodes in exec_list.

I do appreciate the cleverness, but unfortunately it prevents a lot more
cleverness in the form of additional compiler optimizations brought on
by -fstrict-aliasing.

No difference in OglBatch7 (n=20).

Co-authored-by: Davin McCall <davmac@davmac.org>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>

src/compiler/glsl/ir_clone.cpp

@@ -344,7 +344,7 @@ ir_constant::clone(void *mem_ctx, struct hash_table *ht) const
       ir_constant *c = new(mem_ctx) ir_constant;
 
       c->type = this->type;
-      for (exec_node *node = this->components.head
+      for (const exec_node *node = this->components.get_head_raw()
 	      ; !node->is_tail_sentinel()
 	      ; node = node->next) {
 	 ir_constant *const orig = (ir_constant *) node;