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/ast.h

@@ -346,8 +346,8 @@ public:
 
    bool is_single_dimension() const
    {
-      return this->array_dimensions.tail_pred->prev != NULL &&
-             this->array_dimensions.tail_pred->prev->is_head_sentinel();
+      return this->array_dimensions.get_tail_raw()->prev != NULL &&
+             this->array_dimensions.get_tail_raw()->prev->is_head_sentinel();
    }
 
    virtual void print(void) const;