scop detection: remove an iteration over all uses

reenabled reverted patch after checking that it passes without regressions on
the nightly test-suite.  Added testcase from Tobi.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@185720 91177308-0d34-0410-b5e6-96231b3b80d8