Introduce the ScopExpander as a SCEVExpander replacement

  The SCEVExpander cannot deal with all SCEVs Polly allows in all kinds
  of expressions. To this end we introduce a ScopExpander that handles
  the additional expressions separatly and falls back to the
  SCEVExpander for everything else.

Reviewers: grosser, Meinersbur

Subscribers: #polly

Differential Revision: http://reviews.llvm.org/D12066

llvm-svn: 245288