[NVPTX] Disable vector registers

Vectors were being manually scalarized by the backend.  Instead,
let the target-independent code do all of the work.  The manual
scalarization was from a time before good target-independent support
for scalarization in LLVM. However, this forces us to specially-handle
vector loads and stores, which we can turn into PTX instructions that
produce/consume multiple operands.

llvm-svn: 174968