[mlir] Generalize OpFoldResult usage in ops with offsets, sizes and operands.

This revision starts evolving the APIs to manipulate ops with offsets, sizes and operands towards a ValueOrAttr abstraction that is already used in folding under the name OpFoldResult.

The objective, in the future, is to allow such manipulations all the way to the level of ODS to avoid all the genuflexions involved in distinguishing between values and attributes for generic constant foldings.

Once this evolution is accepted, the next step will be a mechanical OpFoldResult -> ValueOrAttr.

Differential Revision: https://reviews.llvm.org/D95310