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

Also #KT-7479 Fixed

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

@@ -0,0 +1,19 @@
+fun baz(s: String?): String {
+    // If String type is given explicitly, problem disappears
+    val t = if (s == null) {
+        ""
+    }
+    else {
+        val u: String? = null
+        if (u == null) return ""
+        // !! is detected as unnecessary here
+        u
+    }
+    return <!DEBUG_INFO_SMARTCAST!>t<!>
+}
+
+fun foo(s: String?): String {
+    if (s == null) return ""
+    val t = if (s == "abc") <!DEBUG_INFO_SMARTCAST!>s<!> else "xyz"
+    return t
+}