[Decompiler Task] Ensure safety of assignment optimizations

# 🚀 Feature Request

Currently, the decompiler involves an assignment optimization. Basically, it transforms the following pattern
```Move
t1 = exp1;
...
use(t1) // t1 is not used anywhere else
```

to 

```Move
...
use(exp1)
```

This is not absolutely safe, because the result of `exp1` could change between `t1=exp1` and `use(t1)`. Further checks are needed to ensure safety.
