Add ScopDetection::isValidRegion(Region)
Check scops a second time before working on them

In rare cases the modification of one scop can effect the validity of other
scops, as code generation of an earlier scop may make the scalar evolution
functions derived for later scops less precise. The example that triggered this
patch was a scop that contained an 'or' expression as follows:

  %add13710 = or i32 %j.19, 1
    -->  {(1 + (4 * %l)),+,2}<nsw><%for.body81>

Scev could only analyze the 'or' as it knew %j.19 is a multiple of 2. This
information was not available after the first scop was code generated (or
independent-blocks was run on it) and SCEV could not derive a precise SCEV
expression any more. This means we could not any more code generate this SCoP.
My current understanding is that there is always the risk that an earlier code
generation change invalidates later scops.  As the example we have seen here is
difficult to avoid, we use this occasion to guard us against all such
invalidations.

This patch "solves" this issue by verifying right before we start working on
a detected scop, if this scop is in fact still valid. This adds a certain
overhead. However the verification we run is anyways very fast and secondly
it is only run on detected scops. So the overhead should not be very large. As
a later optimization we could detect scops only on demand, such that we need
to run scop-detections always only a single time.

This should fix the single last failure in the LLVM test-suite for the new
scev-based code generation.