[MLIR][Slicing] Add utils for computing slices.

[MLIR][Slicing] Add utils for computing slices.

This CL adds tooling for computing slices as an independent CL.
The first consumer of this analysis will be super-vector materialization in a
followup CL.

In particular, this adds:
1. a getForwardStaticSlice function with documentation, example and a
standalone unit test;
2. a getBackwardStaticSlice function with documentation, example and a
standalone unit test;
3. a getStaticSlice function with documentation, example and a standalone unit
test;
4. a topologicalSort function that is exercised through the getStaticSlice
unit test.

The getXXXStaticSlice functions take an additional root (resp. terminators)
parameter which acts as a boundary that the transitive propagation algorithm
is not allowed to cross.

PiperOrigin-RevId: 222446208