mode-switching: Simplify recording of transparency

mode-switching: Simplify recording of transparency

For a given block, an entity is either transparent for
all modes or for none.  Each update to the transparency set
therefore used a loop like:

		for (i = 0; i < no_mode; i++)
		  clear_mode_bit (transp[bb->index], j, i);

This patch instead starts out with a bit-per-block bitmap
and updates the main bitmap at the end.

This isn't much of a simplification on its own.  The main
purpose is to simplify later patches.

gcc/
	* mode-switching.cc (optimize_mode_switching): Initially
	compute transparency in a bit-per-block bitmap.