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

[Nathan Gauër via llvm-commits]

================
@@ -525,6 +525,12 @@ size_t PartialOrderingVisitor::GetNodeRank(BasicBlock *BB) const {
       continue;
 
     auto Iterator = BlockToOrder.end();
+    // This block hasn't been ranked yet. Ignoring.
+    // This doesn't happen often, but when dealing with irreducible CFG, we have
+    // to rank nodes without knowing the rank of all their predecessors.
+    if (Iterator == BlockToOrder.end())
----------------
Keenuts wrote:

They dont't, and I need better test coverage. Somehow I messed up and added this at the wrong place. But somehow all my tests now pass, which means either I lack coverage, or the code below is useless.

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