[mlir][Affine] Fix -lower-affine -debug assertion

[c-rhodes]

Summary

The following:

mlir-opt -lower-affine -debug mlir/test/Conversion/AffineToStandard/lower-affine-to-vector.mlir

triggers an assertion:

affine.for mlir-opt:
mlir/include/mlir/IR/OpDefinition.h:839:
mlir::Block
*mlir::OpTrait::SingleBlock<mlir::AffineForOp>::getBody(unsigned int)
[ConcreteType = mlir::AffineFo
rOp]: Assertion `!region.empty() && "unexpected empty region"' failed.

Since the IR is in an invalid state after AffineForOp is lowered to
scf::ForOp in AffineForLowering.

Bail out in AffineForOp::print if the region is empty.

[sgrechanik]

We are trying to fix this in a general way by running verification before printing and switching to generic op printing on failure, here: https://reviews.llvm.org/D117834

[c-rhodes]

In D120057#3332515, @sgrechanik wrote:

We are trying to fix this in a general way by running verification before printing and switching to generic op printing on failure, here: https://reviews.llvm.org/D117834

Thanks for pointing that out, the issue did seem more fundamental and this fix felt like a band-aid, but your patch addresses that. I'll abandon this change.

It might be worth mentioning on the bug report you're working on a fix: https://github.com/llvm/llvm-project/issues/50530

mlir/lib/Dialect/Affine/IR/AffineOps.cpp

 }
 
 void AffineForOp::print(OpAsmPrinter &p) {
+  // Operation may be invalidated after applying a rewrite pattern.
+  if (region().empty()) {
+    p << " <NO REGION - INVALID>";
+    return;
+  }
+
   p << ' ';
   p.printOperand(getBody()->getArgument(0));
   p << " = ";