Refine collapsing of equal type bounds in OrderingConstraint

`ConstraintHandling#addOneBound` uses `constraint.replace`
when it detects the two bounds of a constraint are equal.

`ConstraintHandling#replace` then recursively replaces the parameter
in the bounds of all other constraints where it appears,
which can transform other constraint entries to also have equal bounds.

This PR applies the same collapsing logic for these transitive updates.

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/core/OrderingConstraint.scala

@@ -708,7 +708,9 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
       def replaceParamIn(other: TypeParamRef) =
         val oldEntry = current.entry(other)
         val newEntry = oldEntry.substParam(param, replacement) match
-          case tp: TypeBounds => current.validBoundsFor(other, tp)
+          case tp: TypeBounds => current.validBoundsFor(other, tp) match
+            case TypeBounds(lo, hi) if lo eq hi => lo
+            case tp => tp
           case tp => tp
         current = boundsLens.update(this, current, other, newEntry)
         var oldDepEntry = oldEntry