[X86][SSE] Auto upgrade PADDUS/PSUBUS intrinsics to UADD_SAT/USUB_SAT generic intrinsics (llvm)

Now that we use the generic ISD opcodes, we can use the generic intrinsics directly as well. This fixes the poor fast-isel codegen by not expanding to an easily broken IR code sequence.

I'm intending to deal with the signed saturation equivalents as well.

Clang counterpart: https://reviews.llvm.org/D55879

Differential Revision: https://reviews.llvm.org/D55855

llvm-svn: 349630