[exegesis] SnippetRepetitor: Don't access liveness if we disabled it

Otherwise, we get:
```
llvm-exegesis: /repositories/llvm-project/llvm/lib/CodeGen/MachineBasicBlock.cpp:1628: MachineBasicBlock::livein_iterator llvm::MachineBasicBlock::livein_begin() const: Assertion `getParent()->getProperties().hasProperty( MachineFunctionProperties::Property::TracksLiveness) && "Liveness information is accurate"' failed.
```