[MLIR][Affine] Add test pass for affine isContiguousAccess (#82923)

`isContiguousAccess` is an important affine analysis utility but is only
tested very indirectly via passes like vectorization and is not exposed.
Expose it and add a test pass for it that'll make it easier/feasible to
write test cases.  This is especially needed since the utility can be
significantly enhanced in power, and we need a test pass to exercise it
directly.

This pass can in the future be used to test the utility of invariant
accesses as well.