i24120 Separate MatchReducer

[Lluc24]

Closes #24120: Refactor: Move MatchReducer to its own file

• Decoupled File Structure: MatchReducer has been moved from TypeComparer.scala to a dedicated file: compiler/src/dotty/tools/dotc/core/MatchReducer.scala. The code movement is isolated in its own commit.
• Inheritance Rationale: The suggested refactoring to composition over inheritance was blocked because MatchReducer requires modifying the mutable protected state (caseLambda) of its superclass, TypeComparer, before invoking dependent inherited methods.
• Cleanup: Removed the originating TODO comment from TypeComparer.scala.
• Git History & Blame:
  • Why not .git-blame-ignore-revs? I did not add this commit to the ignore list because that mechanism forces Git to look at the file state immediately before the commit. Since MatchReducer.scala did not exist before this commit, Git hits a "dead end" and cannot trace the history back to TypeComparer.scala.
  • GitHub Limitation: The GitHub Web UI does not run git blame -C (copy detection) and will incorrectly display me as the author of the moved lines. To verify the original authors (Martin Odersky, etc.), please run the following locally:

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

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

// compiler/src/dotty/tools/dotc/core/MatchReducer.scala, line 328
def matchLegacyPatMat(spec: MatchTypeCaseSpec.LegacyPatMat): MatchResult =
 val caseLambda = constrained(spec.origMatchCase, ast.tpd.EmptyTree)._1.asInstanceOf[HKTypeLambda]
 this.caseLambda = caseLambda
 ...

Mutable protected state caseLambda:

// compiler/src/dotty/tools/dotc/core/ConstraintHandling.scala, line 44
** Potentially a type lambda that is still instantiatable, even though the constraint
*  is generally frozen.
*/
protected var caseLambda: Type = NoType

[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).