Fix PR rtl-optimization/58295

[Eric Botcazou]

> The patch does indeed fix unsigned-extend-1.c on arm and bootstraps
> succesfully on arm-none-linux-gnueabihf.

Thanks for the testing.

> Thanks for fixing this, that test failure has been spoiling my test runs for
> quite a while now :)

You're welcome.  Now applied on mainline and 4.8 branch, after testing on 
PowerPC/Linux for the latter.

-- 
Eric Botcazou