[polly][GPGPU] Fixup related to overloading exponent type in llvm.powi

Commit 4c7f820b changed the llvm.powi intrinsic to support
different 'int' sizes for the exponent. That happened to break
the IntrinsicToLibdeviceFunc mapping in PPCGCodeGeneration, which
obviously should have been updated as part of commit 4c7f820b
(https://reviews.llvm.org/D99439).

The shortcoming was found by buildbots that use
   -DPOLLY_ENABLE_GPGPU_CODEGEN=ON

This patch should fixup the problem.