[mlir][memref] Fix crash in DimOp::fold for subview with ambiguous dropped dims (#183995)
When a rank-reducing subview has multiple size-1 source dimensions and
all strides are dynamic, `computeMemRefRankReductionMask` cannot
unambiguously determine which dimension is dropped. It would fall back
to an arbitrary choice that could be wrong.
In `DimOp::fold`, the code then maps a result dimension to that
(possibly incorrect) source dimension and asserts
`isDynamicSize(sourceIndex)`. If `getDroppedDims()` picked the wrong
dropped dim, the source dim at `sourceIndex` may be statically sized,
causing the assertion to fire.
Fix this by replacing the assert with a guard that returns `{}` (no
fold) when the resolved source dimension is not dynamic. This is safe
because the fold already handles the statically-sized result case
earlier.
Fixes #111244
Assisted-by: Claude Code
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