[MLIR][Affine] Fix/complete access index invariance, add isInvariantAccess (#84602)

isAccessIndexInvariant had outdated code and didn't handle IR with
multiple
affine.apply ops, which is inconvenient when used as a utility.  This is
addressed by switching to use the proper API on AffineValueMap. Add
mlir::affine::isInvariantAccess exposed for outside use and tested via
the test pass. Add a method on AffineValueMap.  Add test cases to
exercise simplification and composition for invariant access analysis.

A TODO/FIXME has been added but this issue existed before.