[X86][SSE] Add PACKUS support to combineVectorTruncation

Similar to the existing code to lower to PACKSS, we can use PACKUS if the input vector's leading zero bits extend all the way to the packed/truncated value.

We have to account for pre-SSE41 targets not supporting PACKUSDW

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317315 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed