Remove use of llvm.codegen intrinsic for GPGPU codegen

We use llvm.codegen intrinsic to generate code for embedded LLVM-IR
strings. The reason we introduce such a intrinsic is that previous
clang/opt tools was NOT linked with various LLVM targets and their
AsmParsers and AsmPrinters. Since clang/opt been linked with all the
needed libraries, we no longer need the llvm.codegen intrinsic.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@211573 91177308-0d34-0410-b5e6-96231b3b80d8