Make OP_ASGN faster.

I think that `*=` and `/=` can be compiled to `OP_MUL` and `OP_DIV`.

Sample `test.rb`:
```ruby
a = 10.0
10000000.times do
  a *= 1
end
```

Without this patch:
```
% time ./bin/mruby test.rb
2.028u 0.003s 0:02.03 99.5%	0+0k 0+0io 0pf+0w
```

With this patch:
```
% time ./bin/mruby test.rb
1.476u 0.003s 0:01.48 99.3%	0+0k 0+0io 0pf+0w
```