isAccessIndexInvariant had outdated code and didn't handle IR with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.
`isContiguousAccess` is an important affine analysis utility but is only
tested very indirectly via passes like vectorization and is not exposed.
Expose it and add a test pass for it that'll make it easier/feasible to
write test cases. This is especially needed since the utility can be
significantly enhanced in power, and we need a test pass to exercise it
directly.
This pass can in the future be used to test the utility of invariant
accesses as well.
