Separate `MatchReducer` from `TypeComparer`?

[mbovel]

There is a to-do on the MatchReducer written by @odersky:

scala3/compiler/src/dotty/tools/dotc/core/TypeComparer.scala

Lines 3580 to 3584 in b7cae74

	/** A type comparer for reducing match types.
	* TODO: Not sure this needs to be a type comparer. Can we make it a
	* separate class?
	*/
	class MatchReducer(initctx: Context) extends TypeComparer(initctx) {

As warm-up tasks for #22587, could we:

• Avoid making MatchReducer a subclass of TypeComparer? Maybe prefer composition over inheritance?
• Move MatchReducer to its own file?
• Improve the documentation of MatchReducer?

cc @Lluc24