[mlir][linalg] Use getAsOpFoldResult in padding (NFC).

After padding, we introduce a ExtractSliceOp to get the final unpadded result. This revision uses getAsOpFoldResult to compute the size of the unpadded result, which guarantees the result type has a partially static shape if some of the sizes of the unpadded result are statically known. At the moment, we rely on canonicalization to cleanup the types after padding.

Depends On D114085

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D114153