Allow switch instructions in SCoPs

  This patch allows switch instructions with affine conditions in the
  SCoP. Also switch instructions in non-affine subregions are allowed.
  Both did not require much changes to the code, though there was some
  refactoring needed to integrate them without code duplication.

  In the llvm-test suite the number of profitable SCoPs increased from
  135 to 139 but more importantly we can handle more benchmarks and user
  inputs without preprocessing.

Differential Revision: http://reviews.llvm.org/D13200

llvm-svn: 248701