Fix nullability propagation in inline class type mapping

#KT-27096

See https://jetbrains.slack.com/archives/C06E082M6/p1537949572000100

compiler/testData/codegen/box/inlineClasses/nullableWrapperEquality.kt

@@ -0,0 +1,18 @@
+// !LANGUAGE: +InlineClasses
+// IGNORE_BACKEND: JVM_IR
+
+inline class Z1(val x: String)
+inline class ZN(val z: Z1?)
+inline class ZN2(val z: ZN)
+
+fun zap(b: Boolean): ZN2? = if (b) null else ZN2(ZN(null))
+
+fun eq(a: Any?, b: Any?) = a == b
+
+fun box(): String {
+    val x = zap(true)
+    val y = zap(false)
+    if (eq(x, y)) throw AssertionError()
+
+    return "OK"
+}