-frerun-loop

[Jeffrey A Law]

It should be noted that rerunning the loop optimizer seems to
interact nicely with the simplify_giv_expr patch.

The simplify_giv_expr patch can potentially create many insns
one loop level out of a loop with complex GIVs.

Re-running loop allows many of those new insns to be hoisted
out of another loop nest.

I don't know how much gain this is in practice, but it may
be worth playing with.

jeff