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
Fixed divide-by-zero crash for issue 84

We can't pass a constant zero as a divisor when building KLEE expressions,
because KLEE invokes llvm::APInt's div functions, which crash on zero value
divisors.

To fix this, we check if the Divisor is a zero constant. If it is, then
we record this UB and simply return a KLEE constant zero rather than going
through KLEE's api(s) such as SDivExpr::create.
tweaks on comments
Fixed an issue on UB conditions
fixed other cases where divisors can be zero