[SPIR-V] Replace assert with report_fatal (#118617)

Irreducible must always be rejected, not only in debug builds. Signed-off-by: Nathan Gauër <brioche@google.com>

Author: Keenuts

llvm/lib/Target/SPIRV/SPIRVUtils.cpp

@@ -580,8 +580,9 @@ size_t PartialOrderingVisitor::visit(BasicBlock *BB, size_t Unused) {
 
  if (!CanBeVisited(BB)) {
  ToVisit.push(BB);
- assert(QueueIndex < ToVisit.size() &&
- "No valid candidate in the queue. Is the graph reducible?");
+ if (QueueIndex >= ToVisit.size())
+ llvm::report_fatal_error(
+ "No valid candidate in the queue. Is the graph reducible?");
  QueueIndex++;
  continue;
  }