K2: Fix generic inference case with !! synthetic call

Result of the `checkNotNull` calls should always be a non-nullable
values.
The simplest idea how to acheive it is adding not-nullable Any bound
to the type parameter declaration.

Existing comment stating about impossibility of such bound seems to be
not 100% correct because it doesn't take into account presence of
definitely-non-nullable X & Any types that allow described case with
nullable generic.

^KT-55804 Fixed

compiler/testData/diagnostics/tests/inference/exclExclInference.kt

@@ -0,0 +1,17 @@
+// FIR_DUMP
+// ISSUE: KT-55804
+
+fun foo() {
+    val x: String?
+    x = materialize()!! // Should be treated as non-nullable assignment
+    <!DEBUG_INFO_SMARTCAST!>x<!>.length // Should be allowed
+}
+
+fun <E> materialize(): E = TODO()
+
+fun <F> test(f: F) = f!!
+
+fun main() {
+    test<String>("").length
+    test<String?>(null).length // `.length` should be allowed because return type of "test" should be inferred to `F & Any`
+}