[PR44610] don't delegitimize MEM from base without offset

[Eric Botcazou]

> This patch prevents the delegitimization when the offset is unknown, so
> that we don't get bogus equivalences.
>
> Regstrapped on x86_64-linux-gnu, i686-linux-gnu and ia64-linux-gnu.
>
> Ok for trunk?

OK for trunk and 4.5 branch.

-- 
Eric Botcazou