X86: Add an SSE2 lowering for 64 bit compares when pcmpgtq (SSE4.2) isn't available.

This pattern started popping up in vectorized min/max reductions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179797 91177308-0d34-0410-b5e6-96231b3b80d8