threading jumps makes niter changed from INTEGER_CST to chrec_dont_know

[Eric Fisher]

Hi,

I found that sometimes -fno-tree-dominator-opts will bring a big speed
promotion. This is because that pass_dominator tries to thread jumps.
But sometimes this will cause that the loop's exit bb does not
dominator its latch bb again. Then pass_complete_unroll is unable to
know the exact number of loop iteration and will not unroll the loop
completely again

Any comments?

Thanks,
Eric.