[X86] Refactored IsSplatVector to use switch. NFCI.
[X86] Refactored IsSplatVector to use switch. NFCI.

Initial step towards making the function more generic (and probably move into SelectionDAG).

This is necessary to avoid massive codegen bloat for PR38243 (Add modulo rotate support to LowerRotate).

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@348498 91177308-0d34-0410-b5e6-96231b3b80d8

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