psi2ir: Fix implicit casts generation

Implicit cast from T to S is not required if
  T <: S.makeNullable()

compiler/testData/ir/irText/expressions/implicitCastToNonNull.kt

@@ -0,0 +1,15 @@
+fun test1(x: String?) =
+    if (x == null) 0 else x.length
+
+fun <T : CharSequence?> test2(x: T) =
+    if (x == null) 0 else x.length
+
+inline fun <reified T : CharSequence?> test3(x: Any) =
+    if (x !is T) 0 else x.length
+
+inline fun <reified T : CharSequence> test4(x: Any?) =
+    if (x !is T) 0 else x.length
+
+fun <T : S?, S> test5(x: T, fn: (S) -> Unit) {
+    if (x != null) fn(x)
+}