[X86] Add example of 'zero shift' guards on rotation patterns (PR34924)
[X86] Add example of 'zero shift' guards on rotation patterns (PR34924)

Basic pattern that leaves an unnecessary select on a rotation by zero result. This variant is trivial - the more general case with a compare+branch to prevent execution of undefined shifts is more tricky.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@338833 91177308-0d34-0410-b5e6-96231b3b80d8

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