[NVPTX] declare no vector registers

[NVPTX] declare no vector registers

Summary:
Without this patch, LoopVectorizer in certain cases (see loop-vectorize.ll)
produces code with complex control flow which hurts later optimizations. Since
NVPTX doesn't have vector registers in LLVM's sense
(NVPTXTTI::getRegisterBitWidth(true) == 32), we for now declare no vector
registers to effectively disable loop vectorization.

Reviewers: jholewinski

Subscribers: jingyue, llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D11089

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241884 91177308-0d34-0410-b5e6-96231b3b80d8