fix opt/2960

[David Edelsohn]

	Before the "don't copy loop headers" change, doloop worked.
Richard's recent comment suggests that the change does not necessarily
promote better, smaller code, which I agree.  Richard tried to fix doloop
by removing the "execute half of the loop one more time" trick, which
seems to be causing more breakage.

	May I suggest that we simply revert the loop headers change, which
Richard wants to do in any case, and add a comment about the fragile
doloop code without trying to fix anything?  Nothing was broken before we
started changing things.  Let's not try to fix something which isn't
broken and leave that effort to masochists.

	Hopefully 3.4 will have a cfg-based loop optimizer and we can toss
all of this code instead of pulling on this string when there are more
important things to be working on.

Thanks, David