perf($compile): only use document fragments when necessary

[jbedard]

This mainly just explains the TODO(perf). I'm sure the actual impact is negligible, but will avoid creating fragments in the majority of cases (where only one node is being replaced).

(this is a subset of #9365 that can be done on it's own)

[lgalfaso]

I am not sure about this change. I see that there might be some performance improvements, but it will also break calling parent.children when there is only one element. I see that this is a very edge case, but people might be using this

[jbedard]

Really it's just for groupScan. Once a node has been replaced I don't think anything is supported on that node? Doesn't mean people don't use it though. I could change this to just update the comment instead?

[jbedard]

Note that if #12094 merges as is then this PR won't be possible anymore (since #12094 always makes use of the document fragment).

[lgalfaso]

I think it is better to land #12094. Later, if there is a way to remove the document fragment, then I am sure that @jbedard will find a way :)

[petebacondarwin]

Closing in favour of #12094