[mlir][Analysis] Guard data flow analysis from no block function

[Lewuathe]

Summary

Foo analysis for testing the data flow analysis does not support the region without any block. Although that analysis is assumed to be used for testing purpose, it is generally better to be explicit about the scope the framework supports.

The original issue was reported here.
https://github.com/llvm/llvm-project/issues/60580

[Lewuathe]

Add a link to the original issue.

[springerm]

mlir/test/lib/Analysis/TestDataFlowFramework.cpp

99

nit: I think getBlocks().empty() is more efficient.

mlir/test/Analysis/invalid.mlir

+// RUN: mlir-opt -split-input-file -pass-pipeline='builtin.module(func.func(test-foo-analysis))'  %s -verify-diagnostics
+
+// -----
+
+// expected-error @+1 {{expected at least one block in the region}}
+func.func private @no_block_func_declaration() -> ()
+

mlir/test/lib/Analysis/TestDataFlowFramework.cpp

struct TestFooAnalysisPass
   void runOnOperation() override;
 };
 } // namespace
 
 LogicalResult FooAnalysis::initialize(Operation *top) {
   if (top->getNumRegions() != 1)
     return top->emitError("expected a single region top-level op");
 
+  if (top->getRegion(0).getBlocks().size() < 1)
+    return top->emitError("expected at least one block in the region");
+
   // Initialize the top-level state.
   getOrCreate<FooState>(&top->getRegion(0).front())->join(0);
 
   // Visit all nested blocks and operations.
   for (Block &block : top->getRegion(0)) {
     visitBlock(&block);
     for (Operation &op : block) {
       if (op.getNumRegions())