Fix PR15296

- Move SRA/SRL/SHL lowering support from DAG combination to DAG lowering
  to support extended 256-bit integer in AVX but not AVX2.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177478 91177308-0d34-0410-b5e6-96231b3b80d8