Speed up .nn

Some tests by Matt had cases where `.nn` figured prominently in the flamegraph.
I optimized it so that the fast path is streamlined and inlined.

In the following test code:
```

  def x: String | Null = "abc"
  val y = x.nn
```
the new implementation is
```
      10: getstatic     #20                 // Field MODULE$:LTest$;
      13: invokevirtual #24                 // Method x:()Ljava/lang/String;
      16: astore_0
      17: getstatic     #29                 // Field scala/runtime/Scala3RunTime$.MODULE$:Lscala/runtime/Scala3RunTime$;
      20: aload_0
      21: invokevirtual #33                 // Method scala/runtime/Scala3RunTime$.nn:(Ljava/lang/Object;)Ljava/lang/Object;
      24: checkcast     #35                 // class java/lang/String
      27: putstatic     #37                 // Field y:Ljava/lang/String;
```
The previous implementation was two bytes shorter, but contained in the critical path

 - A call to a ScalaRuntime method, which was not inlineable by the JIT compiler due to its size,
 - A conversion of a cmparison to a Boolean value (not sure this matters)
 - A cast from the return type `Object` to the actual type.

Author: odersky

library/src/scala/runtime/Scala3RunTime.scala

@@ -14,9 +14,16 @@ object Scala3RunTime:
    *
    *  Extracted to minimize the bytecode size at call site.
    */
+  @deprecated("use Predef.nn instead", "3.2")
   def nn[T](x: T | Null): x.type & T =
     val isNull = x == null
-    if (isNull) throw new NullPointerException("tried to cast away nullability, but value is null")
+    if isNull then nnFail()
     else x.asInstanceOf[x.type & T]
 
+  /** Called by the inline extension def `nn`.
+   *
+   *  Extracted to minimize the bytecode size at call site.
+   */
+  def nnFail(): Nothing =
+    throw new NullPointerException("tried to cast away nullability, but value is null")
 end Scala3RunTime

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -46,7 +46,8 @@ object Predef:
    *  }}}
    */
   extension [T](x: T | Null) inline def nn: x.type & T =
-    scala.runtime.Scala3RunTime.nn(x)
+    if x.asInstanceOf[Any] == null then scala.runtime.Scala3RunTime.nnFail()
+    x.asInstanceOf[x.type & T]
 
   extension (inline x: AnyRef | Null)
     /** Enables an expression of type `T|Null`, where `T` is a subtype of `AnyRef`, to be checked for `null`