Include terminators in instance size estimate
For example, drop glue generated for struct below, doesn't have any
statements, only terminators. Previously it received an estimate of 0,
the new estimate is 13 (6+5 drop terminators, +1 resume, +1 return).
```rust
struct S {
a: String,
b: String,
c: String,
d: String,
e: String,
f: String,
}
```
Originally reported in https://github.com/rust-lang/rust/issues/69382#issue-
569392141