i965/cfg: Make DO instruction begin a basic block.

i965/cfg: Make DO instruction begin a basic block.

The DO instruction doesn't exist on Gen6+. Since before this commit, DO
always ended a basic block, if it also happened to start one (e.g., a
while loop inside an if statement) the block containing only the DO
would actually contain no hardware instructions.

Pre-Gen6's WHILE instructions jumps to the instruction following the DO,
so strictly speaking we won't be modeling that properly, but I claim
there is actually no functional difference.

This will simplify an upcoming change where we want to mark the first
hardware instruction in the loop as beginning a block, and the last
instruction before the loop as ending one.

Reviewed-by: Eric Anholt <eric@anholt.net>