codegen.c: more peephole optimization.

`a=10; a+=1` to generate:

```
    1 000 OP_LOADI      R1      11              ; R1:a
```

instead of:

```
    1 000 OP_LOADI      R1      10              ; R1:a
    1 003 OP_MOVE       R2      R1              ; R1:a
    1 006 OP_ADDI       R2      1
    1 009 OP_MOVE       R1      R2              ; R1:a
```

1 files changed, 21 insertions, 2 deletions

diff --git a/mrbgems/mruby-compiler/core/codegen.c b/mrbgems/mruby-compiler/core/codegen.c
index a3a73fafa..b49db0e31 100644
--- a/mrbgems/mruby-compiler/core/codegen.c
+++ b/mrbgems/mruby-compiler/core/codegen.c
@@ -575,6 +575,9 @@ genjmp2(codegen_scope *s, mrb_code i, uint16_t a, uint32_t pc, int val)
 
 #define genjmp2_0(s,i,a,val) genjmp2(s,i,a,JMPLINK_START,val)
 
+static mrb_bool get_int_operand(codegen_scope *s, struct mrb_insn_data *data, mrb_int *ns);
+static void gen_int(codegen_scope *s, uint16_t dst, mrb_int i);
+
 static void
 gen_move(codegen_scope *s, uint16_t dst, uint16_t src, int nopeep)
 {
@@ -622,6 +625,24 @@ gen_move(codegen_scope *s, uint16_t dst, uint16_t src, int nopeep)
         genop_2SS(s, data.insn, dst, i);
       }
       break;
+    case OP_ADDI: case OP_SUBI:
+      if (nopeep || addr_pc(s, data.addr) == s->lastlabel || data.a != src || data.a < s->nlocals) goto normal;
+      struct mrb_insn_data data0 = mrb_decode_insn(mrb_prev_pc(s, data.addr));
+      if (data0.insn != OP_MOVE || data0.a != data.a || data0.b != dst) goto normal;
+      s->pc = addr_pc(s, data0.addr);
+      /* constant folding */
+      data0 = mrb_decode_insn(mrb_prev_pc(s, data0.addr));
+      mrb_int n;
+      if (data0.a == dst && get_int_operand(s, &data0, &n)) {
+        if ((data.insn == OP_ADDI && !mrb_int_add_overflow(n, data.b, &n)) ||
+            (data.insn == OP_SUBI && !mrb_int_sub_overflow(n, data.b, &n))) {
+          s->pc = addr_pc(s, data0.addr);
+          gen_int(s, dst, n);
+          break;
+        }
+      }
+      genop_2(s, data.insn, dst, data.b);
+      break;
     default:
       goto normal;
     }
@@ -726,8 +747,6 @@ get_int_operand(codegen_scope *s, struct mrb_insn_data *data, mrb_int *n)
   }
 }
 
-static void gen_int(codegen_scope *s, uint16_t dst, mrb_int i);
-
 static void
 gen_addsub(codegen_scope *s, uint8_t op, uint16_t dst)
 {