[flang] [llvm] [clang-tools-extra] [libcxx] [compiler-rt] [mlir] [libc] [clang] [mlir][Linalg] Support dynamic tiles in `lower_pack` transform (PR #76003)

[via cfe-commits]

srcarroll wrote:

> I'm -1 on using `tensor.reshape` op. IMO, we should only use tensor.expand/collapse_shape; they work much better with existing transformations.
> 
> Out of curiosity, what use case do you have in mind? Why do we lower fully dynamic pack op? If it is at high level graph level, we can just use `tensor.pack` which carries more meaningful information. If it is at low level stage (e.g., around vectorization), I think the inner tile sizes should already be resolved to static values? In this context, we can still use `tensor.expand_shape`. It supports the case where one dynamic extent can be expanded into a single dynamic extent and other static extents (e.g., `? -> ?x4`).

I'll admit I dont know the use cases here. I worked on the `lower_unpack` transform to support dynamic sizes because someone in discord said they needed it. And then i saw the NYI comment for `lower_pack` so thought I'd do it
```
"non-static shape NYI, needs a more powerful tensor.expand_shape op"
```
If you never want to support dynamic tiles, then fine by me. But this shouldn't be a NYI comment if you never intend to support it.

https://github.com/llvm/llvm-project/pull/76003