[BUG] WholestageCodegen shows as unsupported when children are removed

[amahussein]

Describe the bug

Currently this is how we decide whther wholStage exec is supported:

    // if any of the execs in WholeStageCodegen supported mark this entire thing
    // as supported
    val anySupported = childNodes.exists(_.isSupported == true)
    val unSupportedExprsArray = childNodes.filter(_.unsupportedExprs.nonEmpty ).map(
      x => x.unsupportedExprs).flatten.toArray
    // average speedup across the execs in the WholeStageCodegen for now

There is a corner case when WholestageCodegen contains an exec like ColumnarToRow.
As a result the WholestageCodegen is set to unsupported but the problem that the ColumnarToRow is supposed to be removed.
In that case, the WholestageCodegen should either be set to shouldRemove or it should be marked as supported because it is theoretically empty.

[tgravescs]

Agree, we should just mark it as should remove like we do when its not in a wholestagecodegen. If that is only exec in it we could just remove the entire wholestagecode gen. That does bring up an interesting question though, that is if cpu has this transition, it might completely go away on GPU if everything is staying columnar. That could make the speedup better. For the qual tool speedup calculation if we remove it we will likely spread that time across the other execs. I don't think we have any logic complex enough to deal with that very well right now.