Add a new instruction OP_LOADI16.

Which loads 16bit integer to the register. The instruction number should
be reorder on massive instruction refactoring. The instruction is added
for `mruby/c` which had performance issue with `OP_EXT`. With this
instruction, `mruby/c` VM can just raise errors on `OP_EXT` extension
instructions.

src/codedump.c

@@ -130,6 +130,10 @@ codedump(mrb_state *mrb, mrb_irep *irep)
       printf("OP_LOADI\tR%d\t-%d\t", a, b);
       print_lv_a(mrb, irep, a);
       break;
+    CASE(OP_LOADI16, BS):
+      printf("OP_LOADI16\tR%d\t%d\t", a, (int)(int16_t)b);
+      print_lv_a(mrb, irep, a);
+      break;
     CASE(OP_LOADI__1, B):
       printf("OP_LOADI__1\tR%d\t\t", a);
       print_lv_a(mrb, irep, a);

src/vm.c

@@ -1047,6 +1047,11 @@ RETRY_TRY_BLOCK:
       NEXT;
     }
 
+    CASE(OP_LOADI16, BS) {
+      SET_INT_VALUE(regs[a], (mrb_int)(int16_t)b);
+      NEXT;
+    }
+
     CASE(OP_LOADSYM, BB) {
       SET_SYM_VALUE(regs[a], syms[b]);
       NEXT;