[Relay][Pass] Separate out the graph partitioning code from fuse_ops.cc

[Hzfengsy]

The current fuse_ops.cc contains the following parts:

1. IndexedForwardGraph and DominatorTree which are used for graph partitioning
2. A Relay Expr visitor to create the DominatorTree
3. A Relay Expr mutator to fuse the ops

This PR separates the graph partitioning code from fuse_ops.cc and moves it to the analysis folder, for:

1. Better code organization and readability as the graph partitioning code is quite long and not directly related to the fusion mutator
2. Possible reuse opportunities for other fusion passes in Relax

NOTE: we won't bring relax fusion in main branch for now, but this pr is still reasonable for main.

cc @tqchen

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[junrushao]

looks like the only dependency to relay is op_attr_types, so shall we instead move them to src/ir/?

[tqchen]

I think it is a good positive change overall. We can think about future reorganizations as followup. Perhaps some extra namespace for related support data structures. Going to merge since this is a positive internal refactor.