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

[Nathan Gauër via llvm-commits]

Keenuts wrote:

> Does it make sense to add the reproducer from #116692, or those changes in the PR covers the reproducer's role in full?

I added 2 versions of the reproducer in the tests:
 - 1 with the exact CFG (minus phi instructions, which doesn't matter for dominance)
 - 1 with the reduced CFG up to the bad pattern (to pin-point a specific pattern)

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