Fix bugs in DMA generation and FlatAffineConstraints; add more test

cases.

- fix bug in calculating index expressions for DMA buffers in certain cases
  (affected tiled loop nests); add more test cases for better coverage.
- introduce an additional optional argument to replaceAllMemRefUsesWith;
  additional operands to the index remap AffineMap can now be supplied by the
  client.
- FlatAffineConstraints::addBoundsForStmt - fix off by one upper bound,
  ::composeMap - fix position bug.
- Some clean up and more comments

PiperOrigin-RevId: 222434628