Fix PR16807

- Lower signed division by constant powers-of-2 to target-independent
  DAG operators instead of target-dependent ones to support them better
  on targets where vector types are legal but shift operators on that
  types are illegal. E.g., on AVX, PSRAW is only available on <8 x i16>
  though <16 x i16> is a legal type.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@192721 91177308-0d34-0410-b5e6-96231b3b80d8