Currently, `GeneralizePadOpPattern` is grouped under
`populatePadOpVectorizationPatterns`. However, as noted in #111349, this
transformation "decomposes" rather than "vectorizes" `tensor.pad`. As
such, it functions as:
* a vectorization _pre-processing_ transformation, not
* a vectorization transformation itself.
To clarify its purpose, this PR turns `GeneralizePadOpPattern` into a
standalone transformation by:
* introducing a dedicated `populateDecomposePadPatterns` method,
* adding a `apply_patterns.linalg.decompose_pad` Transform Dialect Op,
* removing it from `populatePadOpVectorizationPatterns`.
In addition, to better reflect its role, it is renamed as "decomposition"
rather then "generalization". This is in line with the recent renaming
of similar ops, i.e. tensor.pack/tensor.unpack Ops in #116439.
This pass is to handle computationally complex operations like
tensor.pad which are not simply lowered to the exact same operation in
the memref dialect.
Differential Revision: https://reviews.llvm.org/D125384
