[FIR] Properly handle when subject expression in DFA of equality calls

^KT-50785 Fixed

compiler/fir/analysis-tests/testData/resolve/smartcasts/literalInWhenSubject.kt

@@ -0,0 +1,19 @@
+// ISSUE: KT-50785
+
+fun test_1(s: String?) {
+    when (true) {
+        (s != null) -> s.length
+        else -> null
+    }
+}
+
+fun test_2(s: String?) {
+    when (s != null) {
+        true -> s.length
+        else -> null
+    }
+}
+
+fun test_3(s: String?) {
+    if (true == (s != null)) s.length
+}