[mlir][sparse] Factoring out Transforms/CodegenUtils.{cpp,h}

This moves a bunch of helper functions from `Transforms/SparseTensorConversion.cpp` into `Transforms/CodegenUtils.{cpp,h}` so that they can be reused by `Transforms/Sparsification.cpp`, etc.

See also the dependent D115010 which cleans up some corner cases in this change.

Reviewed By: aartbik, rriddle

Differential Revision: https://reviews.llvm.org/D115008