Move scalar immediate shift lowering into a dedicated func

- no functionality change



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177476 91177308-0d34-0410-b5e6-96231b3b80d8
Mark all variable shifts needing customizing

- Prepare moving logic from DAG combining into DAG lowering. There's no
  functionality change.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177477 91177308-0d34-0410-b5e6-96231b3b80d8
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