[ScopDetection] If a loop is not part of a scop, none of it backedges can be

This patch makes sure that in case a loop is not fully contained within a region
that later forms a SCoP, none of the loop backedges are allowed to be part of
the region. We currently do not support the situation where only some of a loops
backedges are part of a scop. Today, this can break both scop modeling and code
generation. One such breaking test case is for example
test/ScopDetectionDiagnostics/loop_partially_in_scop-2.ll, where we totally
forgot to code generate some of the backedges. Fortunately, it is commonly not
necessary to support these partial loops, it is way more common that either
no backedge is included in a region or all loop backedge are included.

This fixes a recent miscompile in
MultiSource/Benchmarks/MiBench/consumer-typeset which was exposed after
r306477.

llvm-svn: 308113