Invoking PrintReplacementLHS for dumping all replacements

Because RHS is NULL after we switched to infer mode,
we need to use PrintReplacementLHS rather than PrintReplacement.
Otherwise, souper pass would crash by passing -souper-debug-level=2.
Merge pull request #100 from chenyang78/debug_level

Invoking PrintReplacementLHS for dumping all replacements