Low overhead looping support

[Michael Hayes]

Jeffrey A Law writes:
 > I _think_ I was referring to loops which are entered via a jump to the exit
 > test.   I didn't see anything which prevented us from trying to use a
 > low overhead loop for these cases.  So we have to make sure to
 > handle it.

Yes, these loops don't have a VTOP note and I think I correctly handle 
them.

BTW, I've rewritten most of this code and am in the process of
rewriting a ChangeLog entry.

Michael.