NVPTX: Remove the legacy ptx intrinsics

- Rename the ptx.read.* intrinsics to nvvm.read.ptx.sreg.* - some but
  not all of these registers were already accessible via the nvvm
  name.
- Rename ptx.bar.sync nvvm.bar.sync, to match nvvm.bar0.

There's a fair amount of code motion here, but it's all very
mechanical.

llvm-svn: 274769