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
