feat: plan for register allocation + temp fix for 'E' not being printed

TODO.md

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+
+0. Define how many physical regs are available, which are caller-saved and callee-saved
+1. Implement if-else and while statements
+2. Find out what use[B] and def[B] are for each block
+3. Define successors and predecessors of each block and make them accessible
+4. Compute liveIn[B] and liveOut[B] for each block
+  
+  Classic equations:
+
+  liveIn[B] = use[B] ∪ (liveOut[B] - def[B])
+
+  liveOut[B] = ⋃ liveIn[S] for S in succ[B]
+5. Linear scan register allocation
+
+
+- Instruction numbering + per-instruction liveness
+- Live intervals
+- Linear scan allocator with ~6 fake regs
+- Spill to stack slots
+- Add “CALL clobbers regs” rule