[RFC combine] PR48308 - Fix issue with missing(?) LOG_LINKS

[Eric Botcazou]

> We then go and check if i1_feeds_into_i2 and that check relies on the
> LOG_LINKS being up-to-date. We find that i2 can be combined with i3
> *but* we've then gone and made a transformation that results in Y
> being used but miss out emitting the set of Y.
>
> The attached patch appears to fix the problem for the reduced testcase
> and reduced^2 testcase attached to PR48308.
>
> Unfortunately this doesn't fix the testcase from PR50313 which
> prima-facie was a dup of this bug which I'm still investigating. This
> has survived bootstrap on x86_64 and is running tests there ( though
> based on a quick reading of the x86 backend I couldn't find any
> parallels of that form) and is still undergoing a full bootstrap run
> on an ARM board.
>
> Thoughts ?

The un-combining thing around line 2800 is somewhat of a kludge and I wouldn't 
be surprised if there were other fallouts.  But your change is clearly correct 
and looks relatively safe, so OK for trunk and 4.6 branch after full testing.

-- 
Eric Botcazou