combiner fix, PR45966

[Bernd Schmidt]

The combine-4 patch caused PR45966.  We have a 4->2 combination, and i0
feeds i1 which feeds i2.  When substituting, we only check i0_feeds_i2
and i1_feeds_i2; we also need to check the i0_feeds_i1/i1_feeds_i2 chain
and substitute accordingly.

Bootstrapped and regression tested on i686-linux; ok?


Bernd
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