DFA: count null comparison as identity comparison #KT-16538 Fixed

+ minor parameter refactoring

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

@@ -0,0 +1,25 @@
+fun foo(x: String?) = x
+
+class Test
+
+class TestWithEquals {
+    override fun equals(other: Any?) = super.equals(other)
+}
+
+fun bar(i: Test?) {
+    if (i == null) foo(<!DEBUG_INFO_CONSTANT!>i<!>)
+}
+
+fun bar(i: TestWithEquals?) {
+    if (i == null) foo(<!DEBUG_INFO_CONSTANT!>i<!>)
+    if (null == i) foo(<!DEBUG_INFO_CONSTANT!>i<!>)
+    when (i) {
+        null -> foo(<!DEBUG_INFO_CONSTANT!>i<!>)
+    }
+}
+
+fun gav(i: TestWithEquals?, j: TestWithEquals?) {
+    if (j == null) {
+        if (i == <!DEBUG_INFO_CONSTANT!>j<!>) foo(<!DEBUG_INFO_CONSTANT!>i<!>)
+    }
+}