R600: Add dag combine for copy of an illegal type.

This helps avoid redundant instructions to unpack, and repack
the vectors. Ideally we could recognize that pattern and eliminate
it. Currently v4i8 and other small element type vectors are scalarized,
so this has the added bonus of avoiding that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213031 91177308-0d34-0410-b5e6-96231b3b80d8