scop detection: check flag before expensive call

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@184000 91177308-0d34-0410-b5e6-96231b3b80d8
scop detection: inline and remove isValidBasicBlock

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@184001 91177308-0d34-0410-b5e6-96231b3b80d8