NFC - fix name / comments - isAccessInvariant

- the name was misleading; this is really checking if a Value being used
  to index was loop IV invariant. Update comment.

- the method is only used locally; what can be exposed in the future is
  isAccessInvariant(LoadOrStoreOp op, Value *iv)

Signed-off-by: Uday Bondhugula <uday@polymagelabs.com>

Closes tensorflow/mlir#285

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/285 from bondhugula:quickfix fe5837abe987980c4ab469a9aa7de8e4f0007d9f
PiperOrigin-RevId: 283771923