[mlir][MemRef][Transform] Don't apply multibuffer on "useless" allocs

`alloc`s that have users outside of loops are guaranteed to fail in
`multibuffer`.

Instead of exposing ourselves to that failure in the transform dialect,
filter out the `alloc`s that fall in this category.

To be able to do this filtering we have to change the `multibuffer`
transform op from `TransformEachOpTrait` to a plain `TransformOp`. This is
because `TransformEachOpTrait` expects that every successful `applyToOne`
returns a non-empty result.

Couple of notes:
- I changed the assembly syntax to make sure we only get `alloc` ops as
  input. (And added a test case to make sure we reject invalid inputs.)
- `multibuffer` can still fail pretty easily when you know its limitations.
  See the updated `op failed to multibuffer` test case for instance.
  Longer term, instead of leaking/coupling the actual implementation (in
  this case the checks normally done in `memref::multiBuffer`) with the
  transform dialect (the added check in `::a...