[SelectionDAG] Remove code from PromoteIntRes_CONCAT_VECTORS that was added in r320674 to help X86.
AVX512 used to promote v32i1 to v32i8 during legalization when BWI was disabled. So this code was added to improve legalization of v32i1 concat_vectors of v16i1 by extending the v16i1 to v16i8 to avoid scalarization.
X86 has since switched to legalizing v32i1 by splitting to v16i1 instead. This has rendered this code unnecessary and its no longer exercised.