Further refactoring varreplacer -- delayedEnqueue enqueueing
Fixing a bug and cleaning up stats update from varreplacer

impl_tmp_stats was clear()-ed and then it was used to update global
stats. Ooops.
Invert order of replaces. Doesn't change semantic, but is easier to read
A cleaner lit1&lit2 definition in varreplacer
Refectoring replace() method in VarReplacer

* Using a helper function to perform checks
* Removing outdated comment