[X86] Simplify some of the type checks in combineSubToSubus.
[X86] Simplify some of the type checks in combineSubToSubus.

If we have SSE2 we can handle any i8/i16 type and let
type legalization deal with it.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@368538 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm