Fix #9058: the sorting function's if-statements for implicit lookup are 'total'

'Total' in the sense of explicitly specifying all possible cases. See https://stackoverflow.com/a/11441813 for a similar error.

Author: anatoliykmetyuk

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

@@ -1174,9 +1174,7 @@ trait Implicits { self: Typer =>
         if (ownerScore < 0) return false
         val arity1 = sym1.info.firstParamTypes.length
         val arity2 = sym2.info.firstParamTypes.length
-        if (arity1 < arity2) return true
-        if (arity1 > arity2) return false
-        false
+        arity1 < arity2
       }
 
       /** Sort list of implicit references according to `prefer`.