irep->pool[n].type should be IREP_TT_*, not MRB_TT_*

fix warning:

```
src/load.c:114:34: warning: implicit conversion from enumeration type 'enum mrb_vtype' to different enumeration type 'enum irep_pool_type' [-Wenum-conversion]
            irep->pool[i].type = MRB_TT_FLOAT;
                               ~ ^~~~~~~~~~~~
```