[llvm] [Utils] Consolidate `LockstepReverseIterator` into own header (NFC) (PR #116657)

[Antonio Frighetto via llvm-commits]

antoniofrighetto wrote:

What occurred here seems quite weird: original commit 585631e554f60b15fd83e240ea8dc4a157a057d0 as part of this PR correctly drops the class in SimplifyCFG, but the landed commit (123dca9b56e1359d8ec7771ea3bd0afd4b1ea6af) merged by David seems to have the LockstepReverseIterator class restored in SimplifyCFG. I was assuming something went wrong while merging this, however, unbeknownst to me, this happened in 48a6df36040d59a4807b0fa9ac46c343900e5225 too, despite being rebased to main. Should have been fixed separately in 93b263a01cf898e609d3896edd0ce95789491bcd.

https://github.com/llvm/llvm-project/pull/116657