[X86][SSE] Don't duplicate Lower256IntArith functionality in LowerShift. NFC.
[X86][SSE] Don't duplicate Lower256IntArith functionality in LowerShift. NFC.

LowerShift was using the same code as Lower256IntArith to split 256-bit vectors into 2 x 128-bit vectors, so now we just call Lower256IntArith.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@264403 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm