[IndexAnalysis] Use mlir::AffineMap instead of IndexingMap for RTVars.
[IndexAnalysis] Use mlir::AffineMap instead of IndexingMap for RTVars.

If we want to compose the indexing map within RTVar with the indexing map of `hlo` that it is associated with, we can just construct an indexing map using the domain of the IndexingMap to which the RTVar belongs to.
