[NVPTX] Auto-upgrade some NVPTX intrinsics to LLVM target-generic code.
Specifically, we upgrade llvm.nvvm.:
These either map directly to an existing LLVM target-generic
intrinsic or map to a simple LLVM target-generic idiom.
In all cases, we check that the code we generate is lowered to PTX as we
These builtins don't need to be backfilled in clang: They're not
accessible to user code from nvcc.
Subscribers: majnemer, cfe-commits, llvm-commits, jholewinski
Differential Revision: https://reviews.llvm.org/D28793