[rtl] combine a vec_concat of 2 vec_selects from the same vector

[Eric Botcazou]

> I successfully regtested the attached trivial patch on x86_64-linux-gnu:
> 
> 2013-03-16  Marc Glisse  <marc.glisse@inria.fr>
> 
>  	* simplify-rtx.c (simplify_binary_operation_1) <VEC_CONCAT>:
>  	Restrict the transformation to equal modes.

OK for 4.9 and 4.8.1 if you add a comment line about the restriction.

-- 
Eric Botcazou