Implement basic optimizations

[jserv]

With the inclusion of SSA in the middle-end, it is now time to implement some common optimizations on the new SSA-based IR. These include constant folding, copy propagation, and dead code elimination.

[jserv]

After @vacantron implements the constant folding optimization, this task will be considered complete, signifying that shecc achieves the goal of implementing a minimal optimizing C compiler capable of compiling itself with typical optimizations.

[vacantron]

Should we abjust the directory structure that making src/ssa.c focus on building the SSA and create another src/optimizer.c to include the existent src/peephole.c and the other optimizers ?

[jserv]

Should we abjust the directory structure that making src/ssa.c focus on building the SSA and create another src/optimizer.c to include the existent src/peephole.c and the other optimizers ?

Up to you. I expect the availability of basic optimizers existing with minimal efforts.