[MLIR] Fix incorrect slice contiguity inference in `vector::isContiguousSlice` (#142422)

Previously, slices were sometimes marked as non-contiguous when they
were actually contiguous. This occurred when the vector type had leading
unit dimensions, e.g., `vector<1x1x...x1xd0xd1x...xdn-1xT>`. In such
cases, only the trailing `n` dimensions of the memref need to be
contiguous, not the entire vector rank.

This affects how `FlattenContiguousRowMajorTransfer{Read,Write}Pattern`
flattens `transfer_read` and `transfer_write` ops.

The patterns used to collapse a number of dimensions equal to the vector
rank which missed some opportunities when the leading unit dimensions of
the vector span non-contiguous dimensions of the memref.
Now that the contiguity of the slice is determined correctly, there is a
choice how many dimensions of the
memref to collapse, ranging from 
a) the number of vector dimensions after ignoring the leading unit
dimensions, up to
  b) the maximum number of contiguous memref dimensions

This patch makes a choice to do minimal memref collapsing. The rationale
behind this decision is that
this way the least amount of information is discarded.

(It follows that in some cases where the patterns used to trigger and
collapse some memref dimensions, after this patch the patterns may
collapse less dimensions).

0 files changed, 0 insertions, 0 deletions