[Polly] Allow multiple reductions in one statement

[Tobias Grosser]

On 27/06/2014 02:10, Johannes Doerfert wrote:
> I changed the first loop nest to perform the task in linear time; the second
> is still as it was.
> I used the summary in the last mail as git commit message (thus I have the
> comment in there).
> Should I also add it to the code?
>
> Is it ok (modulo adding the comment in the code also)?

Here some more comments:

- First of all, the commit message is now outdated. You don't iterate 
over all pairs anymore, no

- Comments in the source code:

   - collectCandiateReductionLoads has little comments. The
     use of Candidates is not commented and it is not clear why more than
     one candidate could be pushed (I understand this, but it may not be
     clear to the reader)

   - You explain that you check for non-overlapping accesses, but it is
     not clear why this is needed and why this is sufficient.

Otherwise, LGTM.

Tobias