Fix codegen of !! for nullable generics

compiler/testData/codegen/box/exclExcl/genericNull.kt

@@ -0,0 +1,12 @@
+fun foo<T>(t: T) {
+    t!!
+}
+
+fun box(): String {
+    try {
+        foo<Any?>(null)
+    } catch (e: Exception) {
+        return "OK"
+    }
+    return "Fail"
+}

compiler/testData/codegen/box/exclExcl/primitive.kt

@@ -0,0 +1,5 @@
+fun box(): String {
+    42!!
+    42.toLong()!!
+    return "OK"!!
+}