RFC: Avoid calling convert_to_mode with invalid rtl.

[Eric Botcazou]

> What do you think of the attached patch ?  Is it suitable for submission
> once the sources are out of stage 4 ?  (No regressions with an
> x86_64-pc-linux-gnu toolchain and several fixes for an rl78-elf toolchain).

Yes, I think it's appropriate for the next stage 1, bearing in mind that there 
is probably a small risk of pessimization on some targets (x32 comes to mind).

-- 
Eric Botcazou