[mlir][sparse] unifying enterLoopOverTensorAtLvl and enterCoIterationOverTensorsAtLvls

The tensor levels are now explicitly categorized into different `LoopCondKind` to instruct LoopEmitter generate different code for different kinds of condition (e.g., `SparseCond`, `SparseSliceCond`, `SparseAffineIdxCond`, etc)

The process of generating a while loop is now dissembled into three steps and they are dispatched to different LoopCondKind handler.
1. Generate LoopCondition (e.g., `pos <= posHi` for `SparseCond`, `slice.isNonEmpty` for `SparseAffineIdxCond`)
2. Generate LoopBody (e.g., compute the coordinates)
3. Generate ExtraChecks (e.g., `if (onSlice(crd))` for `SparseSliceCond`)

Reviewed By: aartbik

Differential Revision: https://reviews.llvm.org/D152464