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

^KT-50785 Fixed

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

@@ -0,0 +1,31 @@
+FILE: literalInWhenSubject.kt
+    public final fun test_1(s: R|kotlin/String?|): R|kotlin/Unit| {
+        when (Boolean(true)) {
+            ==($subj$, !=(R|<local>/s|, Null(null))) ->  {
+                R|<local>/s|.R|kotlin/String.length|
+            }
+            else ->  {
+                Null(null)
+            }
+        }
+
+    }
+    public final fun test_2(s: R|kotlin/String?|): R|kotlin/Unit| {
+        when (!=(R|<local>/s|, Null(null))) {
+            ==($subj$, Boolean(true)) ->  {
+                R|<local>/s|.R|kotlin/String.length|
+            }
+            else ->  {
+                Null(null)
+            }
+        }
+
+    }
+    public final fun test_3(s: R|kotlin/String?|): R|kotlin/Unit| {
+        when () {
+            ==(Boolean(true), !=(R|<local>/s|, Null(null))) ->  {
+                R|<local>/s|.R|kotlin/String.length|
+            }
+        }
+
+    }

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
+}