[mlir][linalg] Control dimensions to pad.

This revision supports padding only a subset of the iteration dimensions via an additional padding-dimensions parameter. This control allows us to pad an operation in multiple steps. For example, one may want to pad only the output dimensions of a producer matmul fused into a consumer loop nest, before tiling and padding its reduction dimension.

Depends On D122309

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D122560