[NVPTX] Use addrspacecast instead of target-specific intrinsics in NVPTXGenericToNVVM.

Summary:
NVPTXGenericToNVVM was using target-specific intrinsics to do address
space casts.  Using the addrspacecast instruction is (a lot) simpler.
But it also has the advantage of being understandable to other passes.
In particular, InferAddrSpaces is able to understand these address space
casts and remove them in most cases.

Reviewers: tra

Subscribers: jholewinski, sanjoy, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D43914

llvm-svn: 326389