Do size modifications after iterator transfer strategy selection

[BillyONeal]

Description

This was the cause of the DevCom reported bug:

https://developercommunity.visualstudio.com/content/problem/739698/vc-163-listsplice-bug.html

where we would choose the incorrect strategy to transfer iterators if and only if the number of transferred iterators was exactly half of the container.

This change replicates internal PR 203902

Checklist:

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