[X86] Emulate AVX 256bit MIN/MAX support by splitting the vector.

In AVX 256bit vectors are valid vectors and therefore the Type Legalizer doesn't
split the VSELECT and SETCC nodes. AVX only supports MIN/MAX on 128bit vectors
and this fix enables vector splitting for this special case in the X86 DAG
Combiner.

This fix is related to PR16695, PR17002, and <rdar://problem/14594431>.

llvm-svn: 191131