[X86] Avoid icmp undef in reduced tests

Because we don't currently simplify icmp with undef in DAG, bugpoint loves to introduce them during reduction.

This is a small step towards re-adding non-undef values into some of the simpler tests so that they should still test correctly and emit similar/same codegen.

Prep work for PR40800 ([SelectionDAG] Add UNDEF handling to SelectionDAG::FoldSetCC).

llvm-svn: 356076