[Refactor][NFC] Map basic blocks to SCoP statements.

  This will simplify the construction of domains and the modeling of
  PHI's.



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@221015 91177308-0d34-0410-b5e6-96231b3b80d8
[Refactor][NFC] Remove unused argument.



git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@221016 91177308-0d34-0410-b5e6-96231b3b80d8