Implement four out of five suggested optimizations (#37)

Fix #27

The remaining suggestion requires understanding where a label point to
so that it can be replaced by its literal equivalent. See #36.

src/chibi.h

@@ -375,6 +375,7 @@ typedef enum {
   EOR2 = EOR | flag_2,
   SFT2 = SFT | flag_2,
 
+  ORAk = ORA | flag_k,
   STA2k = STA2 | flag_k,
   STH2kr = STH2 | flag_k | flag_r,
   POP2r = POP2 | flag_r,

src/optimize.c

@@ -393,6 +393,42 @@ static bool optimize_pass(Instruction *prog, int stage) {
       continue;
     }
 
+    //
+    // https://github.com/lynn/chibicc/issues/27
+    //
+
+    // instances of #00 ORA, [can] be removed.
+    if (IsLit(prog, 0) && prog->next->opcode == ORA) {
+      memcpy(prog, prog->next->next, sizeof(Instruction));
+      changed = true;
+      continue;
+    }
+
+    // [SWP] static arithmetic operation e.g. #abcd SWP, could be #cdab.
+    if (prog->opcode == LIT2 && prog->next->opcode == SWP) {
+      unsigned short tmp = prog->literal;
+      prog->literal = ((tmp & 0xFF) << 8) | ((tmp & 0xFF00) >> 8 );
+      prog->next = prog->next->next;
+      changed = true;
+      continue;
+    }
+
+    // DUP2 #0000 EQU2, [can] be turned into ORAk #00 EQU
+    if (prog->opcode == DUP2 && IsLit2(prog->next, 0) && prog->next->next->opcode == EQU2) {
+      prog            ->opcode = ORAk;
+      prog->next      ->opcode = LIT;
+      prog->next->next->opcode = EQU;
+      changed = true;
+      continue;
+    }
+
+    // #0000 SWP, to #0000
+    if (IsLit2(prog, 0) && prog->next->opcode == SWP) {
+      memcpy(prog->next, prog->next->next, sizeof(Instruction));
+      changed = true;
+      continue;
+    }
+
     prog = prog->next;
   }
   return changed;