RFA: Fix PR39350

[amylaar@spamcop.net]

Quoting Paolo Bonzini <bonzini@gnu.org>:

> Besides this, mixing the behavior with and without movmem in a single
> macro seems to complicate it.  What about instead a CAN_USE_MOVMEM macro and
>
>   if (consider_movmem && CAN_USE_MOVMEM (size, alignment))
>     return true;
>   return MOVE_BY_PIECES (size, alignment);

I don't want to mix up MOVMEM expanders which generate an
expensive function call with ones that generate a move by
pieces in a machine-specific optimized arrangement.

So, CAN_USE_MOVEMEM would either be the wrong macro
(i.e. wrong semantics), or misnamed.  If it is the latter, it
will invariably gravitate to become the former.

What about MOVMEM_BY_PIECES ?