[llvm] [SPIR-V] Fix block sorting with irreducible CFG (PR #116996)

[Vyacheslav Levytskyy via llvm-commits]

VyacheslavLevytskyy wrote:

@Keenuts Let's please proceed with adding a comment explaining what are guarantees of the algorithm and what it tries to achieve, mentioning irreducible case. The PR fixes the issue https://github.com/llvm/llvm-project/issues/116692 so we should proceed but should not lose ideas/asserts from the comments.

https://github.com/llvm/llvm-project/pull/116996