Unaligned block moves and MEM_ALIGN re-broken

[DJ Delorie]

> struct x { char c[8]; };
>  : GET_MODE (RTX) != BLKmode ? GET_MODE_ALIGNMENT (GET_MODE (RTX))      \

I think we're back to the fundamental bug of the mode claiming to be
DI instead of BLK - so it guesses the alignment wrong.  I think
someone needs to somehow be aware of this case, so it can override the
mode-based alignment.