[Polly] Refactor buildScop

Summary: The extracted function buildBBScopStmt will be needed later to be invoked individually on the region's exit block.

Reviewers: grosser, jdoerfert

Subscribers: jdoerfert, llvm-commits, pollydev

Projects: #polly

Differential Revision: http://reviews.llvm.org/D11878

llvm-svn: 244443