[gomp4.1] fold ordered depend(sink) clauses

[Aldy Hernandez <aldyh at redhat dot com>]

The attached patch canonicalizes sink dependence clauses into one folded 
clause if possible (as discussed in the paper "Expressing DOACROSS Loop 
Dependences in OpenMP").

The basic algorithm is to create a sink vector whose first element is 
the GCD of all the first elements, and whose remaining elements are the 
minimum of the subsequent columns.  Further explanations are included in 
the code.

I have also added further warnings/errors for incompatible and 
nonsensical sink offsets.

I suggest you start with the tests and first see if you agree with the 
folded cases.

How does this look?

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