feat: PA4 finished

feat: updated README

Merge pull request #3 from ikitsuchi/pa2

feat: PA2 finished

Merge pull request #2 from ikitsuchi/pa3

feat: PA3 finished

fix: ignore invoke statements

fix: continue traversing when the condition is not a constant

feat: add not dead test case

fix: refactored codes & removed the exit node at the end and passed all unit test cases

fix: handling the case where the RValue of Invoke statement has no side effect

fix: correct logic errors to pass dead assign related test cases

feat: implemented dead code detection

feat: add live variable analysis and constant propagation test cases

feat: implemented live variable analysis and constant propagation