Introduce reduction types

  This change is particularly useful in the code generation as we need
  to know which binary operator/identity element we need to combine/initialize
  the privatization locations.

  + Print the reduction type for each memory access
  + Adjusted the test cases to comply with the new output format and
    to test for the right reduction type



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@212126 91177308-0d34-0410-b5e6-96231b3b80d8