[FIR] Fully expand the bounds of type parameters for canBeNull check

This is required because a not-null bound `Foo` can resolve to a
`typealias Foo = Bar?` in which case we must return true.

#KT-64645 Fixed

compiler/testData/diagnostics/tests/smartCasts/castchecks/kt64645.kt

@@ -0,0 +1,18 @@
+// FIR_IDENTICAL
+// DIAGNOSTICS: -DEBUG_INFO_SMARTCAST, -FINAL_UPPER_BOUND
+// ISSUE: KT-64645
+
+class Box(val value: String)
+typealias NullableBox = Box?
+
+fun <T : NullableBox> foo(x: T) {
+    if (x != null) {
+        x.value
+    }
+}
+
+fun foo2(x: NullableBox) {
+    if (x != null) {
+        x.value
+    }
+}