[MLIR][GPU] Properly model step in parallel loop to gpu conversion.

Summary:
The original patch had TODOs to add support for step computations,
which this commit addresses. The computations are expressed using
affine expressions so that the affine canonicalizers can simplify
the full bound and index computations.

Also cleans up the code a little and exposes the pass in the
header file.

Differential Revision: https://reviews.llvm.org/D75052