[tuples] fix and enable forward propagation

[Aldy Hernandez]

> Also it looks like
> 
>   /* Constants can be always propagated.  */
>   if (is_gimple_min_invariant
>       (rhs_to_tree (TREE_TYPE (gimple_assign_lhs (def_stmt)), def_stmt)))
>     return true;
> 
> asks for a gimple_assign_min_invariant_rhs_p () predicate like

Sorry, I inherited this code, and I obviously missed a few spots.

For the record, I think any use of rhs_to_tree is most likely a bug, or
at least a really inefficient way of doing things.