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.
• 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.
• Blame Handling: Added the commit hash to .git-blame-ignore-revs to ensure accurate git blame history tracking of the original authors.
• Cleanup: Removed the originating TODO comment from TypeComparer.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