[X86][AVX] Fold concat(packus(),packus()) -> packus(concat(),concat()) (PR34773)

[X86][AVX] Fold concat(packus(),packus()) -> packus(concat(),concat()) (PR34773)

Basic "revectorization" combine, we can probably do more opcodes here but it can be a tricky cost-benefit depending on where the subvectors came from - but this case helps shuffle combining.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360134 91177308-0d34-0410-b5e6-96231b3b80d8