[X86] Teach DAG unfoldMemoryOperand to reconvert CMPs to tests

Summary:
Copy MI-level cmp->test conversion to SelectionDAG-level memory unfold.
This fixes a regression from upcoming D41293 change.

Reviewers: craig.topper, RKSimon

Reviewed By: craig.topper

Subscribers: llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D42808

llvm-svn: 324261