55430f8673
We've observed that the SLPVectorizer is too conservative on NVPTX because it over-estimates the cost to build a vector. PTX has a single `mov` instruction that can build e.g. `<2 x half>` vectors from scalars, however the SLPVectorizer over-estimates it as the cost of 2 insert elements. To fix this I customize `getScalarizationOverhead` to lower the cost for building 2x16 types.