Add special handling for comparisons with Singleton types

[odersky]

Fixes #15030

[smarter]

LGTM but this might need equivalent handling in tasty-query /cc @sjrd

[noti0na1]

I don't think the rule in this PR is correct:

class A
class B extends A

def f(a: A, c: A) =
  val b1: a.type = a
  val b2: a.type & B = a.asInstanceOf[a.type & B]
  val b3: c.type & A = c
  val b4: a.type | c.type = c

  val d1: b1.type = a
  val d2: b2.type = a // ok
  val d3: b3.type = a // error
  val d4: b4.type = a // error

In 3.4.2:

> scala-cli compile -S 3.4 Stest.scala
Compiling project (Scala 3.4.2, JVM (21))
[error] ./Stest.scala:324:21
[error] Found:    (a : A)
[error] Required: (b3 : (c : A) & A)
[error]   val d3: b3.type = a // error
[error]                     ^
[error] ./Stest.scala:325:21
[error] Found:    (a : A)
[error] Required: (b4 : (a : A) | (c : A))
[error]   val d4: b4.type = a // error
[error]                     ^
Error compiling project (Scala 3.4.2, JVM (21))
Compilation failed

cc @sjrd