[polly] handle loops using unsigned induction variables

[Sebastian Pop]

Hi Tobi,

Here is a patch that makes polly handle a large set of loops using unsigned
induction variables. These were not handled previously because polly is not able
(yet) to reason about wrap-around of unsigned types.  The patch moves the
detection of loop exit conditions before the checks for other unsigned branches,
letting all the loops using an unsigned induction variable to be handled by the
SCEV trip count analysis.

I understand that this is a short-cut, and an important one to allow polly to
handle a large class of loops.  A longer term solution would be to not use the
SCEV trip count, and handle the loop exit conditions as extra constraints (as
Tobi proposed in another thread).

Until we get there, is the attached patch ok to commit?

Thanks,
Sebastian
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