Remove TODO in MatchReducer

[Lluc24]

Closes #24120: remove the TODO comment.

The TODO comment left by @odersky regarding MatchReducer was:

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

Lines 3597 to 3601 in c9309e7

	/** 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) {

Keep MatchReducer as a TypeComparer

In the end, we do need MatchReducer to be a TypeComparer. The refactoring to composition over inheritance was not possible because MatchReducer requires modifying the mutable protected field caseLambda of its superclass, ConstraintHandling (which is a superclass of TypeComparer), before invoking inherited methods that depends it.

Mutable protected field caseLambda:

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

Lines 44 to 47 in c9309e7

	/** Potentially a type lambda that is still instantiatable, even though the constraint
	* is generally frozen.
	*/
	protected var caseLambda: Type = NoType

MatchReducer modifies caseLambda variable of the superclass, and this modification is later needed when invoking methods of TypeComparer:

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

Lines 3879 to 3881 in c9309e7

	def matchLegacyPatMat(spec: MatchTypeCaseSpec.LegacyPatMat): MatchResult =
	val caseLambda = constrained(spec.origMatchCase, ast.tpd.EmptyTree)._1.asInstanceOf[HKTypeLambda]
	this.caseLambda = caseLambda

Maybe we could revisit this if we get rid of matchLegacyPatMat one day.

Keep MatchReducer in the same file

Moving MatchReducer to its own file creates significant issues with Git history tracking:

• .git-blame-ignore-revs Limitation: This mechanism only works for in-place refactorings. When a file is moved, Git looks at the file state immediately before the commit. Since MatchReducer.scala didn't exist before, Git cannot trace the history back to TypeComparer.scala.

• GitHub UI Limitation: GitHub's web interface does not support git blame -C (copy detection), so the GitHub UI would incorrectly show the person who moved the file as the author of all lines, losing attribution to the original authors.

• Loss of Context: Keeping MatchReducer in the same file as TypeComparer makes it easier to understand their tight coupling and why inheritance is necessary.

• Local Workaround Required: Users would need to run git blame -C locally to see the correct history, which is not ideal for most contributors:

  git blame -C compiler/src/dotty/tools/dotc/core/MatchReducer.scala

[mbovel]

In its current state, it seems the .git-blame-ignore-revs doesn't work as expected:

Maybe it's due to the merge commit? Or maybe this is just the GitHub UI.

I'd try to rebase to avoid the merge commit, and have three commits: 1. move the match reducer, 2. add the move commit to .git-blame-ignore-revs and 3. remove the the todo (with an explanation in the commit description).