Reuse compareAlternatives#prev computation

Author: EugeneFlesselle

compiler/src/dotty/tools/dotc/typer/Implicits.scala

@@ -1330,13 +1330,13 @@ trait Implicits:
         if alt1.ref eq alt2.ref then 0
         else if alt1.level != alt2.level then alt1.level - alt2.level
         else
-          var cmp = comp(using searchContext())
-          if cmp == 0 && alt1.ref.symbol.is(Implicit) && alt2.ref.symbol.is(Implicit) then
+          lazy val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
+          val cmp = comp(using searchContext()) match
             // if we get an ambiguity with new rules for a pair of old-style implicits, fall back to old rules
-            cmp = comp(using searchContext().addMode(Mode.OldImplicitResolution))
+            case 0 if alt1.ref.symbol.is(Implicit) && alt2.ref.symbol.is(Implicit) => prev
+            case cmp => cmp
           val sv = Feature.sourceVersion
           if isWarnPriorityChangeVersion(sv) then
-            val prev = comp(using searchContext().addMode(Mode.OldImplicitResolution))
             if disambiguate && cmp != prev then
               def warn(msg: Message) =
                 val critical = alt1.ref :: alt2.ref :: Nil