[GPGPU] Make sure managed arrays are prepared at the beginning of the scop

Summary:
This resolves some "instruction does not dominate use" errors, as we used to
prepare the arrays at the location of the first kernel, which not necessarily
dominated all other kernel calls.

Reviewers: Meinersbur, bollu, singam-sanjay

Subscribers: nemanjai, pollydev, llvm-commits, kbarton

Differential Revision: https://reviews.llvm.org/D36372

llvm-svn: 310196