Fix regression issue in flex decoding.

[chengjunlu]

The tt.store operation with BlockPointer fallbacks to scatter store if the BLOCK shape or the value layout was not supported by the 2D BLOCK IO.
The lowering code would transform the BlockPointer to the pointers and masks.

The scatter store should apply the and to the maskElems if the llMask doesn't exsits.

[Copilot]

Pull Request Overview

This PR fixes a regression issue in flex decoding by updating block pointer handling and prefetch operations. Key changes include:

• Updating the 2D prefetch shape calculation with a tensor shape bound.
• Replacing an unreachable branch in prefetch conversion with a regular pointer prefetch handler.
• Adjusting mask handling both in prefetch and load/store conversions.

Comments suppressed due to low confidence (1)

third_party/intel/lib/TritonIntelGPUToLLVM/LoadStoreOpToLLVM.cpp:565

• [nitpick] There are multiple debug print statements (e.g., 'johnlu debug:') in production-oriented code; consider removing or gating these under a debug flag to avoid cluttering output during normal execution.

llvm::outs() << "johnlu debug: operands " << (opIdx == DpasEncodingAttr::OpIdx::OperandA ? "A" : "B") << "\n";

[alexbaden]

We need to add a unit test. Maybe to test_core or test_block_loads in the Intel directory?

[etiotto]

Please add a lit test (unit test) to cover this bug.

[alexbaden]

Ok for now. I think I prefer a python test here so we can compare to some known good output, vs the lit test which can be hard to parameterize / generalize to lots of different masks. If you have ideas for improving test_block_load.py to test this condition I would be very interested in hearing them!

[chengjunlu]

Please add a lit test (unit test) to cover this bug.

Enhanced the LIT test case to cover the boundary check condition.

[chengjunlu]

Ok for now. I think I prefer a python test here so we can compare to some known good output, vs the lit test which can be hard to parameterize / generalize to lots of different masks. If you have ideas for improving test_block_load.py to test this condition I would be very interested in hearing them!

I will add the unit test in another PR #3876