[MachineVerifier] Try harder to verify LiveVariables

[foad]

Summary

Verify the LiveVariables analysis after a pass that claims to preserve
it, even if there are no further passes (apart from the verifier itself)
that would use the analysis.

[foad]

Rebase.

[MatzeB]

LGTM

llvm/lib/CodeGen/MachineVerifier.cpp

struct MachineVerifierPass : public MachineFunctionPass {
     const std::string Banner;
 
     MachineVerifierPass(std::string banner = std::string())
       : MachineFunctionPass(ID), Banner(std::move(banner)) {
         initializeMachineVerifierPassPass(*PassRegistry::getPassRegistry());
       }
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
+      AU.addUsedIfAvailable<LiveVariables>();
       AU.setPreservesAll();
       MachineFunctionPass::getAnalysisUsage(AU);
     }
 
     bool runOnMachineFunction(MachineFunction &MF) override {
       // Skip functions that have known verification problems.
       // FIXME: Remove this mechanism when all problematic passes have been
       // fixed.