Remove unused method
Comment out graph coloring in Main
Update reference test output

The only changes to the outputs is the order of Phi operands since the
order now depends on which branch it came from.
Add DefaultValue class

This provides a placeholder for default variable values, which can then
be detected to signal an error, or replaced with 0 to 0 initialize all
variables.
Add pass to replace the default values of vars
Update test output for 0 initialization
Zero initialize right when the variable is parsed

This is instead of using another pass to replace the values.
Update reference outputs

The outputs changed since zero initialization is happening before
constant arithmetic, so more expressions are constant.
Make variable 0 init optional, but default to it
Add reference output for more tests
Fix typo in cse_if test
Recognize built in functions

Generate the correct opcode to 'inline' these functions.
Update reference outputs

Now builtin functions are inlined as a single instruction.
Add Fetch instruction

This instruction will fetch a function argument from wherever it will
exist according to the calling conventions, and put it into a register,
as assigned by the register allocator. This allows the register
allocation to be simpler, but also adds some brain damage to the
generated code.
Generate fetch instructions when parsing functions
Update reference test outputs to account for fetch