[NVPTX] Turn on Loop/SLP vectorization

Since PTX has grown a <2 x half> datatype vectorization has become more
important. The late LoadStoreVectorizer intentionally only does loads
and stores, but now arithmetic has to be vectorized for optimal
throughput too.

This is still very limited, SLP vectorization happily creates <2 x half>
if it's a legal type but there's still a lot of register moving
happening to get that fed into a vectorized store. Overall it's a small
performance win by reducing the amount of arithmetic instructions.

I haven't really checked what the loop vectorizer does to PTX code, the
cost model there might need some more tweaks. I didn't see it causing
harm though.

Differential Revision: https://reviews.llvm.org/D46130

llvm-svn: 331035