[MachineVerifier] Try harder to verify LiveVariables

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.

Differential Revision: https://reviews.llvm.org/D129213

llvm/lib/CodeGen/MachineVerifier.cpp

@@ -294,6 +294,7 @@ namespace {
 
     void getAnalysisUsage(AnalysisUsage &AU) const override {
       AU.addUsedIfAvailable<LiveStacks>();
+      AU.addUsedIfAvailable<LiveVariables>();
       AU.setPreservesAll();
       MachineFunctionPass::getAnalysisUsage(AU);
     }