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/nullableUnit.kt

@@ -0,0 +1,20 @@
+fun isNull(x: Unit?) = x == null
+
+fun isNullGeneric<T : Any>(x: T?) = x == null
+
+fun deepIsNull0(x: Unit?) = isNull(x)
+fun deepIsNull(x: Unit?) = deepIsNull0(x)
+
+fun box(): String {
+    if (!isNull(null)) return "Fail 1"
+
+    val x: Unit? = null
+    if (!isNull(x)) return "Fail 2"
+
+    val y = x
+    if (!isNullGeneric(y)) return "Fail 3"
+
+    if (!deepIsNull((null : Unit?) ?: null)) return "Fail 4"
+
+    return "OK"
+}