[llvm] [DebugInfo][RemoveDIs] Emulate inserting insts in dbg.value sequences (PR #73350)

[Jeremy Morse via llvm-commits]

jmorse wrote:

More mucking around with the CodeGenPrepare patch (uploading shortly) has revealed that actually the supported use case is where the reinserted instruction is placed with `insertAfter` rather than insertBefore, which changes the set of inputs that needs to be supported. The commit just pushed switches this facility to support that (and udpate the tests for it).

I feel it's alright to support this limited situation seeing how it's only for one consumer (CodeGenPrepare). One day codegenprepare will go away anyway... according to the file comment... written in 2008.

https://github.com/llvm/llvm-project/pull/73350