[Arc] Add dominance-aware pass to sink ops and merge scf.if ops

[fabianschuiki]

Add the MergeIfs pass to the Arc dialect. This pass covers a handful of control flow optimizations that are valuable to pick up after hw.modules have been linearized and lowered into arc.model ops:

• It moves operations closer to their earliest user, if possible sinking them into blocks if all uses are nested in the same block.
• It merges adjacent scf.if operations with the same condition.
• It moves operations in between two scf.if operations ahead of the first if op to allow them to be merged.

The MergeIfs pass can operate on SSACFG regions. It assigns an integer order to each operation and considers that order to determine up to which point operations can be moved without moving beyond their first use and without crossing interfering side-effecting ops. The pass is aware of side-effects, and in particular uses the non-aliasing between arc.state and arc.memory to track read/write side-effects at a per-state level, which allows for fairly aggressive optimization. Other side-effecting ops act as a hard barrier and will not be moved or moved over.

This pass supersedes the very effective GroupResetsAndEnables pass we have been using until now. The latter relies on the LegalizeStateUpdate pass to run at a later point however, which will be removed in a future PR, thus making this new pass necessary.

This is a preparatory step for a later PR that overhauls the LowerState pass. That rewrite will make the arc.clock_tree and arc.passthrough ops obsolete, which is why they are not present in the tests.

[TaoBi22]

This is super cool! Very cool that this detects and works with non-overlapping side effects too!

[maerhart]

Awesome work! 🎉