[mlir][inliner] Refactor MLIR inliner pass and utils. (#84059)

This is just code refactoring done as a preparation for adding
MLIR inliner cost model hook(s).
Related discussion: https://discourse.llvm.org/t/inliner-cost-model/2992

The logic of SCC-based MLIR inliner is separated into the Inliner
implementation. The MLIR inliner pass becomes, well, just a pass
that invokes the SCC-based MLIR inliner.