[NFC][RemoveDIs] Use iterators for insertion at various call-sites (#84736)

These are the last remaining "trivial" changes to passes that use
Instruction pointers for insertion. All of this should be NFC, it's just
changing the spelling of how we identify a position.

In one or two locations, I'm also switching uses of getNextNode etc to
using std::next with iterators. This too should be NFC.

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Merged by: Stephen Tozer <stephen.tozer@sony.com>