[X86] Remove GCCBuiltins from cvtsi2ss/cvtsi2sd/cvtss2sd intrinsics as they aren't used by clang. Add TODOs to remove these and some other unused intrinsics.
[X86] Remove GCCBuiltins from cvtsi2ss/cvtsi2sd/cvtss2sd intrinsics as they aren't used by clang. Add TODOs to remove these and some other unused intrinsics.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@286091 91177308-0d34-0410-b5e6-96231b3b80d8

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