Support symbolic operands for memref replacement; fix memrefNormalize

- allow symbols in index remapping provided for memref replacement
- fix memref normalize crash on cases with layout maps with symbols

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

Closes tensorflow/mlir#139

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/139 from bondhugula:memref-rep-symbols 2f48c1fdb5d4c58915bbddbd9f07b18541819233
PiperOrigin-RevId: 269851182