[SPARK-18376][SQL] Skip subexpression elimination for conditional expressions

[viirya]

What changes were proposed in this pull request?

As per discussion at #15807, we should disallow subexpression elimination for expressions wrapped in conditional expressions such as If.

How was this patch tested?

Jenkins tests.

Please review https://cwiki.apache.org/confluence/display/SPARK/Contributing+to+Spark before opening a pull request.

[viirya]

cc @cloud-fan@kiszk

[SparkQA]

Test build #68389 has finished for PR 15824 at commit 548e45f.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

retest this please.

[SparkQA]

Test build #68393 has finished for PR 15824 at commit 548e45f.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

retest this please.

[SparkQA]

Test build #68394 has finished for PR 15824 at commit 548e45f.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[viirya]

Close this for now because we might prefer other solution.