[llvm] [DebugInfo][RemoveDIs] Have getInsertionPtAfterDef return an iterator (PR #73149)

[Jeremy Morse via llvm-commits]

jmorse wrote:

I've pushed up an equivalent patch using `end()`, although there are some downsides: it's possible for users to ignore the "invalid" return-value of end() and insert at end(), although I suppose that'll eventually be caught by the verifier. It's also not always clear what block's end() one should compare the return with code -- hence I've left `getFreezeInsertPt` returning an optional. Thoughts on whether this is better?

https://github.com/llvm/llvm-project/pull/73149