Enable common subexpression elimination by default
Add new test, cse_array2
Add reference outputs for default CSE
Fix typo in easy_loops
Make stores not need registers
Always add Phi nodes to interference graph
Change how interference graph is output

Added a function to main that will output the interference graph for the
whole program.
Compute the interference graph for the program

It isn't used right now though, no register allocation is performed.
Add option to output interference graph