[mlir][spirv] Emit error when block to be unlinked has uses after structurization (#163942)

This fixes the bug where deserializer would fail, with as assert, during
the control flow structurization when a block to be removed still had
uses. This indicates that the block that was sunk still is being
referenced outside the region as the control flow is not structured.

closes #163099

Author: IgWod-IMG

mlir/lib/Target/SPIRV/Deserialization/Deserializer.cpp

@@ -2619,6 +2619,11 @@ LogicalResult ControlFlowStructurizer::structurize() {
   // region. We cannot handle such cases given that once a value is sinked into
   // the SelectionOp/LoopOp's region, there is no escape for it.
   for (auto *block : constructBlocks) {
+    if (!block->use_empty())
+      return emitError(block->getParent()->getLoc(),
+                       "failed control flow structurization: "
+                       "block has uses outside of the "
+                       "enclosing selection/loop construct");
     for (Operation &op : *block)
       if (!op.use_empty())
         return op.emitOpError("failed control flow structurization: value has "