Retested: RFA: fix PR middle-end/23290

[Joern RENNECKE]

Thanks for the review.

Richard Henderson wrote:

>Though I'll state again for the record that any ABI that bases
>its decisions on modes instead of tree codes is broken.
>  
>
The specific mode that was tested against was BLKmode.  If we want to 
make ports
impervious to random use of BLKmode, we should declare the practice of
FUNCTION_ARG yielding a REG rtx as obsolete, i.e. everything but a plain 
stack
argument has to be expressed with a PARALLEL.

At the moment, tm.texi still states that a value not passes on the stack 
is usually
expressed with a REG rtx, and these can't handle BLKmode.