[Refactor] Use only one BlockGenerator for a SCoP

  This change has two main purposes:
    1) We do not use a static interface to hide an object we create and
       destroy for every basic block we copy.
    2) We allow the BlockGenerator to store information between calls to
       the copyBB method. This will ease scalar/phi code generation
       later on.

  While a lot of method signatures were changed this should not cause
  any real behaviour change.

  Differential Revision: http://reviews.llvm.org/D7467

llvm-svn: 228443