Re-run of loop pass

[Michael Hayes <m dot hayes at elec dot canterbury dot ac dot nz>]

What's the philosophy of rerunning the loop optimisation pass twice?

I've noticed that this can generate poorer code when unrolling loops
or using the bct optimisation if a BIV is eliminated on the first
pass.

On the second pass, loop_iterations is often unable to determine the
iteration count for loops with limits that should be able to be
determined at compile time.  Loop unrolling or the bct optimisation
then makes a dog's breakfast of things.

Michael.