Fixed the addnuw, subnuw, mulnsw, mulnuw according to review comments by Peter

1. Implemented addnuw and subnuw by verifying N+1th bit
2. Implemented mulnsw and mulnuw by verifying the extended bits
of multiplication result (i.e. bit 33 to 64) to zero.
3. Commented out the test cases for which souper does not report
any optimization and also commented out the change in hasCandidate()

TODO :
- to fix the point 3 mentioned above.
- to add Klee expressions for all test cases
- To reason about why not call addInst(I); functions and instead
call directly AddExpr::create(...); for all the cases handled. In my
opinion, the function calls are good enough to make the code modular and
uniform.