[X86] Combine zext(packus(x),packus(y)) -> concat(x,y) (PR39637)

[X86] Combine zext(packus(x),packus(y)) -> concat(x,y) (PR39637)

Its proving tricky to combine shuffles across multiple vector sizes, so for now I'm adding this more specific combine - the pattern is common enough to be worth it as a first step.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354757 91177308-0d34-0410-b5e6-96231b3b80d8