[FlattenAlgo] Translate to C++ bindings

Translate the full algorithm to use the new isl C++ bindings

This is a large piece of code that has been written with the Polly IslPtr<>
memory management tool, which only performed memory management, but did not
provide a method interface. As such the code was littered with calls to
give(), copy(), keep(), and take(). The diff of this change should give a
good example how the new method interface simplifies the code by removing the
need for switching between managed types and C functions all the time
and consequently also the need to use the long C function names.

These are a couple of examples comparing the old IslPtr memory management
interface with the complete method interface.

Check properties
----------------

Before:

  if (isl_aff_is_zero(Aff.get()) ||  isl_aff_is_one(Aff.get()))
    return true;

After:

  if (Aff.is_zero() || Aff.is_one())
    return true;

Type conversion
---------------

Before:

  isl_union_pw_multi_aff *UPMA =
      give(isl_union_pw_multi_aff_from_union_map(UMap.copy());

After:

  isl::union_pw_multi_aff UPMA = UMap;

Type construction
-----------------

Before:

  auto Empty = give(isl_union_map_empty(Space.copy());

After:

  auto Empty = isl::union_map::empty(Space);

Operations
----------

Before:

  Set = give(isl_union_set_intersect(Set.copy(), Set2.copy());

After:

  Set = Set.intersect(Set2);

Tags: #polly

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D30617

llvm-svn: 297463