When building equivalence classes, maintain a set of seen Insts

Helps avoid degenerate behavior with large self-referential insts.
Clang's CharInfoTest.cpp is a good test case for this.
Generate candidate queries on demand

The SMTLIB query builder has the same problem that I just fixed in the
equivalence class builder. This at least lets us look at the instructions
by not supplying a solver flag.
Roll KLEE to 5d8db05524f5216900e952c3e1fc2aac9c424391

This incorporates a fix in the SMTLIB printer for arrays with 1-bit ranges.