Transform JuvixReg into SSA form

[lukaszcz]

• Implement a transformation of JuvixReg into SSA form.
• Needed for the translation to Cairo Assembly
• Since there are no loops within a single function, SSA can be computed by going forward in the code of a function and keeping a map from old variables to new variables, updating the map at each assignment.
• In JuvixReg, variables are divided into two groups (VarGroup in Reg.Language): VarGroupArgs and VarGroupLocal. Within each group the variables are indexed by integers. The indices need to be recomputed so that each variable is assigned at most once. Variables in VarGroupArgs are never assigned, so we need to consider only local variables (VarGroupLocal).
• Variables are assigned by instructions which have the _instr*Result field (the field specifies the variable to which the result is assigned).
• All branches in InstrBranch and InstrCase must assign consistent variable indices to variables live after the instruction (used afterwards). It can be assumed that the top of the value stack is the only live variable assigned in each branch. Perhaps one needs to store the information about the index of the top of the value stack after a branching instruction when translating from JuvixAsm to JuvixReg.
• Remember to update the live variable information stored in instructions.