dialects: (linalg) thread loop-carried values through tile loops

[dipo101]

Part of the tensor-based tiling checkbox on #6140, and the second of the smaller PRs split out of #6362. Follows on from #6366.

Tiling an operand with value semantics needs the accumulated result threaded through the tile loop nest, since each tile produces a new value rather than writing through a view.

This gives _build_tile_loops an optional iter_args parameter. The outermost loop initialises the carried values from iter_args, and every nested loop initialises them from the enclosing loop's block arguments, so the innermost body sees the values accumulated by the surrounding iterations:

outer loop:   iter_args(%accI = %B)        <- the original value
  inner loop: iter_args(%accJ = %accI)     <- the enclosing loop's block argument

Initialising a nested loop from the original values instead would silently discard the work of the enclosing iterations, giving IR that verifies but computes the wrong thing. The added test pins that down: removing the reassignment makes it fail on the inner loop's iter_args.

scf.ForOp derives its result types from its iter_args, so the loops gain results automatically.

Tiling memrefs passes no iter_args, leaving the loops without block arguments or results, so its generated IR is unchanged. Nothing passes iter_args yet; that arrives with the later PRs in the series, which is why this one is not reachable from the pass and _build_tile_loops is tested directly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.94%. Comparing base (5484aad) to head (8dcbda1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6368   +/-   ##
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  Coverage   86.94%   86.94%           
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  Files         440      440           
  Lines       65966    65969    +3     
  Branches     7491     7491           
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+ Hits        57355    57358    +3     
  Misses       7035     7035           
  Partials     1576     1576           

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[dipo101]

@superlopuh PR 2 of 5 is ready for review, ptal

[superlopuh]

Nice