Revert to only reducing singleton args in singleton ops

As discussed in #7628

Author: MaximeKjaer

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

@@ -1043,10 +1043,7 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] w
      */
     def compareCompiletimeAppliedType(tp: AppliedType, other: Type, fromBelow: Boolean): Boolean = {
       if (defn.isCompiletime_S(tp.tycon.typeSymbol)) compareS(tp, other, fromBelow)
-      else {
-        val reduced = tp.tryCompiletimeConstantFold.getOrElse(tp.superType)
-        if (fromBelow) recur(other, reduced) else recur(reduced, other)
-      }
+      else tp.tryCompiletimeConstantFold.exists(folded => if (fromBelow) recur(other, folded) else recur(folded, other))
     }
 
     /** Like tp1 <:< tp2, but returns false immediately if we know that

tests/neg/singleton-ops.scala

@@ -5,14 +5,8 @@ object Test {
   summon[1763 =:= 41 * 43]
   summon[2 + 2 =:= 3] // error
   summon[29 * 31 =:= 900] // error
-  summon[Any <:< Int + 1] // error
-  summon[Int + 1 <:< Any]
-
-  val a: Int + Int = 3
-  val c: 1 + Int = 2
-  val d: Int + 1 = 1
-  summon[Int + 1 =:= Int]
-  summon[1 + 1 =:= Int] // error
+  summon[Int <:< Int + 1] // error
+  summon[1 + Int <:< Int]
 
   val t0: 2 + 3 = 5
   val t1: 2 + 2 = 5 // error