FIR: fix contract deserialization for assertIs<T>

compiler/fir/analysis-tests/testData/resolveWithStdlib/smartcasts/assertIs.fir.txt

@@ -0,0 +1,5 @@
+FILE: assertIs.kt
+    public final fun foo(arg: R|kotlin/Any|): R|kotlin/Unit| {
+        R|kotlin/test/assertIs|<R|kotlin/String|>(R|<local>/arg|, String())
+        R|<local>/arg|.R|kotlin/String.length|
+    }

compiler/fir/analysis-tests/testData/resolveWithStdlib/smartcasts/assertIs.kt

@@ -0,0 +1,6 @@
+import kotlin.test.*
+
+fun foo(arg: Any) {
+    assertIs<String>(arg, "")
+    arg.length
+}