docs: Add execution_count to basic_block_t

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

src/IR_DESIGN_DOC.md

@@ -223,5 +223,8 @@ typedef struct
 {
     uint32_t instruction_start_index;
     uint32_t instruction_count;
+
+    // Used to order hot and cold blocks in memory.
+    uint32_t execution_count;
 } basic_block_t;
 ```