GPGPU: Generate PTX assembly code for the kernel modules

GPGPU: Generate PTX assembly code for the kernel modules

Run the NVPTX backend over the GPUModule IR and write the resulting assembly
code in a string.

To work correctly, it is important to invalidate analysis results that still
reference the IR in the kernel module. Hence, this change clears all references
to dominators, loop info, and scalar evolution.

Finally, the NVPTX backend has troubles to generate code for various special
floating point types (not surprising), but also for uncommon integer types. This
commit does not resolve these issues, but pulls out problematic test cases into
separate files to XFAIL them individually and resolve them in future (not
immediate) changes one by one.

git-svn-id: https://llvm.org/svn/llvm-project/polly/trunk@276396 91177308-0d34-0410-b5e6-96231b3b80d8