[X86][AVX] Ensure chained subvector insertions are the same size (PR42833)

Before combining insert_subvector(insert_subvector(vec, sub0, c0), sub1, c1) patterns, ensure that the subvectors are all the same type. On AVX512 targets especially we might have a mixture of 128/256 subvector insertions.

llvm-svn: 367429