[GPGPU] Synchronize after each kernel, not each copy out

Summary:
This change reduces the overall number of synchronize calls for kernels with
a lot of output data at the cost of additional synchronize calls for kernels
launched in sequence without any device to host transfers in between. As the
latter pattern is a lot less frequent, this seems a better tradeoff.

Even though the above motivation would be motivation enough, this is just
a step towards enabling ppcg to not compute to and from device copy calls
at all, which would be incorrect in case we still relied on these calls to
place our synchronization statements.

Reviewers: Meinersbur, bollu, singam-sanjay

Reviewed By: bollu

Subscribers: nemanjai, kbarton, pollydev, llvm-commits

Tags: #polly

Differential Revision: https://reviews.llvm.org/D36867

llvm-svn: 311155