DFA: take into account if / else and when expression assignments #KT-10002 Fixed

Also #KT-7479 Fixed

compiler/testData/diagnostics/tests/smartCasts/whenExprNonNull.kt

@@ -0,0 +1,18 @@
+fun baz(s: String?): String {
+    if (s == null) return ""
+    // if explicit type String is given for t, problem disappears
+    val t = when(<!DEBUG_INFO_SMARTCAST!>s<!>) {
+        // !! is detected as unnecessary here
+        "abc" -> s
+        else -> "xyz"
+    }
+    return <!DEBUG_INFO_SMARTCAST!>t<!>
+}
+
+fun foo(s: String?): String {
+    val t = when {
+        s != null -> s
+        else -> ""
+    }
+    return <!DEBUG_INFO_SMARTCAST!>t<!>
+}