Fix: Add pass manager barrier.

The LNT test suite with -polly-process-unprofitable
-polly-position=before-vectorizer currenty fails 59 tests. With this
barrier added, only 16 keep failing. This is probably because Polly's
code generation currently does not correctly preserve all analyses it
promised to preserve. Temporarily add this barrier until further
investigation.

llvm-svn: 262488