[llvm] [CodeGen] Combine two loops in SlotIndexes.cpp file (PR #127631)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm approved this pull request.

I'd hope for no codegen change, but there are unfortunately a few places that I suppose are relying on exact index numbers 

https://github.com/llvm/llvm-project/pull/127631