add missing memref cast fold pattern for dim op

- add missing canonicalization pattern to fold memref_cast + dim to
  dim (needed to propagate constant when folding a dynamic shape to
  a static one)

- also fix an outdated/inconsistent comment in StandardOps/Ops.td

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

Closes tensorflow/mlir#126

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/126 from bondhugula:quickfix 4566e75e49685c532faffff91d64c5d83d4da524
PiperOrigin-RevId: 269020058