[GPGPU] Do not create copy statements when targetting managed memory

Summary:
They are not used and consequently do not even need to be computed. This reduces
the overall compile time for our kernel from 1m33s to 17s.

Reviewers: Meinersbur, bollu, singam-sanjay

Reviewed By: bollu

Subscribers: nemanjai, pollydev, llvm-commits, kbarton

Tags: #polly

Differential Revision: https://reviews.llvm.org/D36868

llvm-svn: 311157