[SPARK-16928] [SQL] Recursive call of ColumnVector::getInt() breaks JIT inlining

## What changes were proposed in this pull request?

In both `OnHeapColumnVector` and `OffHeapColumnVector`, we implemented `getInt()` with the following code pattern:
```
public int getInt(int rowId) {
if (dictionary == null)
{ return intData[rowId]; }
else
{ return dictionary.decodeToInt(dictionaryIds.getInt(rowId)); }
}
```
As `dictionaryIds` is also a `ColumnVector`, this results in a recursive call of `getInt()` and breaks JIT inlining. As a result, `getInt()` will not get inlined.

We fix this by adding a separate method `getDictId()` specific for `dictionaryIds` to use.

## How was this patch tested?

We tested the difference with the following aggregate query on a TPCDS dataset (with scale factor = 5):
```
select
  max(ss_sold_date_sk) as max_ss_sold_date_sk,
from store_sales
```
The query runtime is improved, from 202ms (before) to 159ms (after).

Author: Qifan Pu <qifan.pu@gmail.com>

Closes #14513 from ooq/SPARK-16928.