[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174968 91177308-0d34-0410-b5e6-96231b3b80d8