docs: Rework variable design implementation

When I was designing the instruction struct, I needed to figure out how
an instruction knows which argument position is a variable and a
constant. So the variable type is baked into the struct.

I also added a variable array for obvious reasons.

Signed-off-by: Ronald Caesar <github43132@proton.me>

src/IR_DESIGN_DOC.md

@@ -6,7 +6,22 @@ All memory will be allocated from a contiguous memory arena before the pass begi
 // Ref: Chapter 3.1.3
 typedef struct
 {
-    instruction_t* defining_instruction;
+    type_t type; // TYPE_INT, TYPE_FLOAT
+
+    // 1: x = 4        Defines x so reachingDef = 1 (x₁)
+    // 2: y = x + 1    Only uses x so reachingDef is still 1.
+    // 3: x = 5        Redefines x so reachingDef = 2 (x₂).
     int reaching_definition;
-} variable_t;
+} original_variable_t;
+
+// This array maps the source code definition to its SSA version.
+// IT is the frontend's job to populate this array.
+//
+// Example:
+//
+// Instruction 100: x = y + z
+//
+// If `x` is at index 5, original_variables[5].reachingDef = 100.
+// `x` maps to instruction 100.
+original_variable_t original_variables[???];
 ```