More aggressive threading causing loop-interchange-9.c regression

[Aldy Hernandez]

> It would be helpful to have the patch causing the issue to look at the IL.
> But as Micha said, there needs to be a perfect loop nest for interchange
> to work.
> 
> Richard.

Absolutely!  I'm attaching the reduced testcase, as well as the patch.

The problematic thread shows up in the thread2 dump:

Checking profitability of path (backwards):  bb:3 (4 insns) bb:9 (0 
insns) bb:5
   Control statement insns: 2
   Overall: 2 insns
   Registering FSM jump thread: (5, 9) incoming edge;  (9, 3)  (3, 8) 
nocopy; (3, 8)

Thanks.
Aldy
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