[DAG] FoldSetCC - add missing icmp(X,undef) -> isTrueWhenEqual case

Followup to D59363 which failed to handle the icmp(X,undef) -> isTrueWhenEqual case - similar to llvm::ConstantFoldCompareInstruction

As discussed on the review, this is affecting some previously reduced test cases, but will also prevent reductions from relying on this inconsistent behaviour in the future.

Differential Revision: https://reviews.llvm.org/D158068