Add isl operator overloads for isl::pw_aff (Try II)

Piecewise affine expressions have directly corresponding mathematical
operators. Introduce these operators as overloads as this makes writing
code with isl::pw_aff expressions more directly readable.

We can now write:

  A = B + C    instead of    A = B.add(C)

Reviewers: Meinersbur, bollu, sebpop

Reviewed By: Meinersbur

Subscribers: philip.pfaffe, pollydev, llvm-commits

Differential Revision: https://reviews.llvm.org/D45534

llvm-svn: 329880