[TableGen] Integrate TableGen-based macro fusion (#73115)

`Fusion` is inherited from `SubtargetFeature` now. Each definition
of `Fusion` will define a `SubtargetFeature` accordingly.

Method `getMacroFusions` is added to `TargetSubtargetInfo`, which
returns a list of `MacroFusionPredTy` that will be evaluated by
MacroFusionMution.

`getMacroFusions` will be auto-generated if the target has `Fusion`
definitions.