[SPARK-18058][SQL][TRIVIAL] Use dataType.sameResult(...) instead equality on asNullable datatypes

## What changes were proposed in this pull request?
This is absolutely minor. PR https://github.com/apache/spark/pull/15595

 uses `dt1.asNullable == dt2.asNullable` expressions in a few places. It is however more efficient to call `dt1.sameType(dt2)`. I have replaced every instance of the first pattern with the second pattern (3/5 were introduced by #15595).

## How was this patch tested?
Existing tests.

Author: Herman van Hovell <hvanhovell@databricks.com>

Closes #16041 from hvanhovell/SPARK-18058.

(cherry picked from commit d449988b)
Signed-off-by: Reynold Xin <rxin@databricks.com>