On x86: (shl V, 1) -> add V,V
Hardware support for vector-shift is sparse and in many cases we scalarize the
result. Additionally, on sandybridge padd is faster than shl.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@143311 91177308-0d34-0410-b5e6-96231b3b80d8
