Add rewrite pattern to compose maps into affine load/stores

- add canonicalization pattern to compose maps into affine loads/stores;
  templatize the pattern and reuse it for affine.apply as well

- rename getIndices -> getMapOperands() (getIndices is confusing since
  these are no longer the indices themselves but operands to the map
  whose results are the indices). This also makes the accessor uniform
  across affine.apply/load/store. Change arg names on the affine
  load/store builder to avoid confusion. Drop an unused confusing build
  method on AffineStoreOp.

- update incomplete doc comment for canonicalizeMapAndOperands (this was
  missed from a previous update).

Addresses issue tensorflow/mlir#121

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

Closes tensorflow/mlir#122

COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/122 from bondhugula:compose-load-store e71de1771e56a85c4282c10cb43f30cef0701c4f
PiperOrigin-RevId: 269619540