[mlir][linalg] Make padding work for rank-reducing slice ops.

Adapt the computation of a static bounding box to take rank-reducing slice operations into account by filtering out reduced size one dimensions. The revision is needed to make padding work for decomposed convolution operations. The decomposition introduces rank reducing extract slice operations that previously let padding fail.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D115336