Add dead store elimination optimization

Implement basic dead store elimination that identifies stores that are
immediately overwritten within the same basic block before any loads occur.

The optimization scans store and global_store instructions and tracks:
- Whether a load occurs before another store to the same location
- If another store overwrites the value before it is used

Author: jserv

src/defs.h

@@ -313,6 +313,7 @@ struct var {
     bool is_ternary_ret;
     bool is_logical_ret;
     bool is_const; /* whether a constant representaion or not */
+    bool is_dead;  /* whether this variable is dead (never used) */
 };
 
 typedef struct {

src/ssa.c

@@ -1525,6 +1525,7 @@ void build_reversed_rpo();
 /* Forward declarations for optimization functions */
 void simple_copy_propagation(basic_block_t *bb);
 void redundant_load_elimination(basic_block_t *bb);
+void dead_store_elimination(basic_block_t *bb);
 
 void optimize(void)
 {
@@ -1543,6 +1544,7 @@ void optimize(void)
             /* Apply optimization passes */
             simple_copy_propagation(bb);
             redundant_load_elimination(bb);
+            dead_store_elimination(bb);
 
             /* instruction level optimizations */
             for (insn_t *insn = bb->insn_list.head; insn; insn = insn->next) {
@@ -1815,3 +1817,38 @@ void redundant_load_elimination(basic_block_t *bb)
         }
     }
 }
+
+
+/* Dead store elimination - remove stores that are never used */
+void dead_store_elimination(basic_block_t *bb)
+{
+    for (insn_t *insn = bb->insn_list.head; insn; insn = insn->next) {
+        if (insn->opcode == OP_store || insn->opcode == OP_global_store) {
+            if (!insn->rd)
+                continue;
+
+            /* Check if this store is immediately overwritten */
+            bool is_dead = true;
+            for (insn_t *next = insn->next; next; next = next->next) {
+                /* If we find a load before another store, it's not dead */
+                if ((next->opcode == OP_load ||
+                     next->opcode == OP_global_load) &&
+                    next->rs1 == insn->rd) {
+                    is_dead = false;
+                    break;
+                }
+                /* If we find another store to the same location, this store is
+                 * dead */
+                if ((next->opcode == OP_store ||
+                     next->opcode == OP_global_store) &&
+                    next->rd == insn->rd) {
+                    break;
+                }
+            }
+
+            if (is_dead && insn->rd) {
+                insn->rd->is_dead = true;
+            }
+        }
+    }
+}