Extend map canonicalization to propagate constant operands

- extend canonicalizeMapAndOperands to propagate constant operands into
  the map's expressions (and thus drop those operands).
- canonicalizeMapAndOperands previously only dropped duplicate and
  unused operands; however, operands that were constants were
  retained.

This change makes IR maps/expressions generated by various
utilities/passes even simpler; also makes some of the test checks more
accurate and simpler -- for eg., 0' instead of symbol(%{{.*}}).

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

Closes tensorflow/mlir#107

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/107 from bondhugula:canonicalize-maps c889a51486d14fbf7db489f224f881e7e1ff7d72
PiperOrigin-RevId: 266085289