Add basic cost modeling to the dialect conversion infrastructure. This initial...

Add basic cost modeling to the dialect conversion infrastructure. This initial cost model favors specific patterns based upon two criteria:
1) Lowest minimum pattern stack depth when legalizing.
  - This leads the system to favor patterns that have lower legalization stacks, i.e. represent a more direct mapping to the target.

2)  Pattern benefit.
  - When considering multiple patterns with the same legalization depth, this favors patterns with a larger specified benefit.

PiperOrigin-RevId: 252713470