[mlir] Improve/fix fusion in the presence of scalable tiles

[banach-space]

This is a meta-task to capture the effort to:

• Enable post-tiling fusion in the presence of scalable tile sizes, especially for linalg.pack, linalg.unpack and linalg.mmt4d.
• Avoid requiring pack/unpack/mmt4d to be split across multiple functions in this PR: [mlir][test] Add e2e test for linalg.mmt4d + SVE #157815 (while keeping tiling and vectorisation enabled).

See also iree-org/iree#21475.

CC @egebeysel

[llvmbot]

@llvm/issue-subscribers-mlir

Author: Andrzej Warzyński (banach-space)

This is a meta-task to capture the effort to enabling post-tiling fusion in the presence of scalable tile sizes, especially for `linalg.pack`, `linalg.unpack` and `linalg.mmt4d`.

See e.g.:
iree-org/iree#21475
#150185