Update dma-generate: update for multiple load/store op's per memref

- introduce a way to compute union using symbolic rectangular bounding boxes
- handle multiple load/store op's to the same memref by taking a union of the regions
- command-line argument to provide capacity of the fast memory space
- minor change to replaceAllMemRefUsesWith to not generate affine_apply if the
  supplied index remap was identity

PiperOrigin-RevId: 230848185