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

[Nathan Gauër via llvm-commits]

Keenuts 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 #116692 so we should proceed but should not lose ideas/asserts from the comments.

I'll send another PR which sorts the IR using a preorder traversal instead. This one will instead add tests to the visitor, and make irreducible case fail and not hang.
Just got side tracked on other tasks, but it's next on my list.

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