[SPARK-16699][SQL] Fix performance bug in hash aggregate on long string keys

In the following code in `VectorizedHashMapGenerator.scala`:
```
    def hashBytes(b: String): String = {
      val hash = ctx.freshName("hash")
      s"""
         |int $result = 0;
         |for (int i = 0; i < $b.length; i++) {
         |  ${genComputeHash(ctx, s"$b[i]", ByteType, hash)}
         |  $result = ($result ^ (0x9e3779b9)) + $hash + ($result << 6) + ($result >>> 2);
         |}
       """.stripMargin
    }

```
when b=input.getBytes(), the current 2.0 code results in getBytes() being called n times, n being length of input. getBytes() involves memory copy is thus expensive and causes a performance degradation.
Fix is to evaluate getBytes() before the for loop.

Performance bug, no additional test added.

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

Closes #14337 from ooq/SPARK-16699.

(cherry picked from commit d226dce1)
Signed-off-by: Reynold Xin <rxin@databricks.com>