[X86][SSE] Use the general SMAX/SMIN/UMAX/UMIN pattern matching and remove the X86 implementation

Follow up to D10947 - D9746 added general SMAX/SMIN/UMAX/UMIN pattern matching to SelectionDAGBuilder::visitSelect.

This patch removes the X86 implementation and improves the AVX1/AVX2 support to correctly lower 256-bit integer vectors.

Differential Revision: http://reviews.llvm.org/D12006

llvm-svn: 244949