Don't treat values of type "Unit?" to be always equal to Unit.VALUE

Two changes here: StackValue.Constant does cast iff value is non-null (if null,
no cast between classes is really needed, as null can be an instance of
anything), and codegen for safe qualified expressions uses correct type for the
expression

 #KT-4265 Fixed

compiler/testData/codegen/box/unit/kt4265.kt

@@ -0,0 +1,14 @@
+fun <T : Any, R> T.let(f: (T) -> R): R = f(this)
+
+fun box(): String {
+    val o: String? = null
+
+    var state = 0
+
+    o?.let {
+        state = 1
+    } ?: ({ state = 2 })()
+
+    if (state != 2) return "Fail: $state"
+    return "OK"
+}