KT-6774 Cannot find equals() when comparing with null

#KT-6774 Fixed

compiler/testData/diagnostics/tests/controlStructures/commonSupertypeOfT.kt

@@ -0,0 +1,30 @@
+// KT-6774 Cannot find equals() when comparing with null
+
+fun <T: Any> fn(t1: T, t2: T?) {
+    val x = if (true) t1 else t2
+    x == null
+    x?.equals(null)
+    x?.hashCode()
+    x.toString()
+    x!!.hashCode()
+
+    val y = t2 ?: t1
+    y == t1
+    y.equals(null)
+    y.hashCode()
+    y.toString()
+    y.hashCode()
+}
+
+trait Tr {
+    fun foo()
+}
+
+fun <T: Tr> fn(t1: T, t2: T?) {
+    val x = if (true) t1 else t2
+    x?.foo()
+    x!!.foo()
+
+    val y = t2 ?: t1
+    y.foo()
+}

compiler/testData/diagnostics/tests/controlStructures/commonSupertypeOfT.txt

@@ -0,0 +1,11 @@
+package
+
+internal fun </*0*/ T : Tr> fn(/*0*/ t1: T, /*1*/ t2: T?): kotlin.Unit
+internal fun </*0*/ T : kotlin.Any> fn(/*0*/ t1: T, /*1*/ t2: T?): kotlin.Unit
+
+internal trait Tr {
+    public open override /*1*/ /*fake_override*/ fun equals(/*0*/ other: kotlin.Any?): kotlin.Boolean
+    internal abstract fun foo(): kotlin.Unit
+    public open override /*1*/ /*fake_override*/ fun hashCode(): kotlin.Int
+    public open override /*1*/ /*fake_override*/ fun toString(): kotlin.String
+}