[SelectionDAG] Make sorting predicate stronger to remove non-deterministic ordering

Summary:
This fixes failures in the following tests uncovered by D39245:
        LLVM :: CodeGen/ARM/ifcvt3.ll
        LLVM :: CodeGen/ARM/switch-minsize.ll
        LLVM :: CodeGen/X86/switch.ll

Reviewers: hans, efriedma

Reviewed By: hans

Subscribers: fhahn, aemerson, kristof.beyls, llvm-commits

Differential Revision: https://reviews.llvm.org/D39995

llvm-svn: 318680