[NVPTX] Infer AS of pointers passed to kernels as integers.

When pointers are passed within aggregates, we sometimes end up with IR that
loads them as integers and the converts them back to pointers. Typically it's
due to a memcpy or SROA. E.g. https://godbolt.org/z/xM3n5daaa

Normally we treat all pointers passed to a CUDA kernel as global pointers and
the same treatment should be applied to the pointers we load/store as integers.

Differential Revision: https://reviews.llvm.org/D142664