Move Pass registration into polly library

This ensures that the polly passes get properly registered both, when using
polly as a loadable module and when directly linking it into clang/opt/bugpoint.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@204255 91177308-0d34-0410-b5e6-96231b3b80d8