fwprop fix for PR79405

[Bernd Schmidt]

We have two registers being assigned to each other:

  (set (reg 213) (reg 209))
  (set (reg 209) (reg 213))

These being the only definitions, we are happy to forward propagate reg 
209 for reg 213 into a third insn, making a new use for reg 209. We are 
then happy to forward propagate reg 213 for it in the same insn... 
ending up in an infinite loop.

I don't really see an elegant way to prevent this, so the following just 
tries to detect the situation (and more general ones) by brute force. 
Bootstrapped and tested on x86_64-linux, verified that the test passes 
with a ppc cross, ok?


Bernd

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