[X86][SSE] Add extra trunc(shl) test cases
[X86][SSE] Add extra trunc(shl) test cases

The existing trunc_shl_17_v8i16_v8i32 test case should (but doesn't) fold to zero, I've added 2 new test cases:
 - trunc_shl_16_v8i16_v8i32 which folds to zero (this is actually testing the target faux shuffle combine)
 - trunc_shl_15_v8i16_v8i32 which should perform the full shl + truncate


git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@334188 91177308-0d34-0410-b5e6-96231b3b80d8

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