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

[Nathan Gauër via llvm-commits]

Keenuts wrote:

Hello all!
This PR now leaves the visitor as-is, and only changes the `sortBlocks()` function to use a reverse port order iterator.
It also adds unit tests distinct from the ones for the visitor, testing block sorting only.

The PR to handle irreducible in the visitor is https://github.com/llvm/llvm-project/pull/117887

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