Enable partial and runtime loop unrolling for NVPTX backend via
TTI::UnrollingPreferences with a small threshold. This partially unrolls
small loops which are often unrolled by the PTX to SASS compiler
and unrolling earlier can be beneficial.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242049 91177308-0d34-0410-b5e6-96231b3b80d8