Question about mem_set_list in propagate_block_info

[Fred Fish]

> It's not really what you want to do either.  That's probably more
> likely to commit the variable to memory than is strictly necessary. 
> Something like
>   union matrix_t { int m __attribute__((__mode__(V16SI))); int x[16]; };

I probably should have mentioned that we no longer use that ugly macro
and now use:

	static inline int
	MatrixElement(matrix_t a, int row, int col)
	{
	  union {
	    matrix_t matrix;
	    int elements[4][4];
	  } u;
	  u.matrix = a;
	  return (u.elements[row][col]);
	}

however the problem was noted back when we used the macro so I decided
to try to get to the bottom of it, in case there actually was a
compiler bug.  If the consensus is that the code is illegal, then I'm
fine with that.

As a matter of curiosity though I'd still be interested in knowing
more about when memory references are supposed to be removed from the
mem_set_list per my original email.

-Fred