dialects: (linalg) write tiles back into the tensors carried by tile loops

[dipo101]

Part of the tensor-based tiling checkbox on #6140.

A tile of a memref is written through the subview that views it, but a tile of a tensor is a new value, so it has to be written back explicitly and carried out of the loops it was computed in.

This passes the outputs of an op with tensor semantics as the loop nest iteration arguments, slices those outputs from the values the innermost loop carries rather than from the originals, and writes each computed tile back with a tensor.insert_slice. The innermost loop yields the updated tensors and each enclosing loop yields the results of the loop nested inside it, so the outermost loop produces the fully updated tensors, which then replace the results of the original op:

%C = scf.for %i = ... iter_args(%accI = %B) -> (tensor<4x4xf32>) {
  %r = scf.for %j = ... iter_args(%accJ = %accI) -> (tensor<4x4xf32>) {
    %in   = tensor.extract_slice %A[%i, %j] ...
    %out  = tensor.extract_slice %accJ[%i, %j] ...     // from the carried value
    %tile = linalg.generic ins(%in) outs(%out) ... -> tensor<2x2xf32>
    %upd  = tensor.insert_slice %tile into %accJ[%i, %j] ...
    scf.yield %upd : tensor<4x4xf32>
  }
  scf.yield %r : tensor<4x4xf32>
}

Two things here are easy to get wrong, and both give IR that verifies while dropping the work of the surrounding iterations: slicing an output from the original tensor rather than from the carried value, and erasing the op rather than replacing its results.

A tile is written back exactly where it was extracted from, so both use the same SliceParameters.

Tiling memrefs carries nothing, writes nothing back and yields nothing, so its generated IR is unchanged and the existing filecheck expectations are untouched. Tiling tensors is still rejected during analysis, so this is not yet reachable from the pass; a later PR lifts that.

[codecov]

Codecov Report

❌ Patch coverage is 82.22222% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.93%. Comparing base (7da3e2a) to head (0e88664).

Files with missing lines	Patch %	Lines
xdsl/dialects/linalg/transforms/tiling.py	82.22%	7 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6370      +/-   ##
==========================================
- Coverage   86.94%   86.93%   -0.02%     
==========================================
  Files         440      440              
  Lines       65973    65999      +26     
  Branches     7493     7493              
==========================================
+ Hits        57360    57376      +16     
- Misses       7036     7044       +8     
- Partials     1577     1579       +2     

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