Replace CHECKCAST kotlin/Unit with ARETURN for tail call optimization

Sometimes instead of {POP, GETSTATIC Unit.INSTANCE, ARETURN} sequence
the codegen emits {CHECKCAST Unit, ARETURN} sequence, which breaks tail
call optimization. By replacing CHECKCAST with ARETURN we eliminate
this issue.

 #KT-19790: Fixed

compiler/testData/codegen/box/coroutines/tailCallOptimizations/whenUnit.kt

@@ -0,0 +1,33 @@
+// WITH_RUNTIME
+// WITH_COROUTINES
+// CHECK_BYTECODE_LISTING
+import helpers.*
+import kotlin.coroutines.experimental.*
+
+sealed class X {
+    class A : X()
+    class B : X()
+}
+
+var log = ""
+
+suspend fun process(a: X.A) { log = "${log}from A;" }
+suspend fun process(b: X.B) { log = "${log}from B;" }
+
+suspend fun process(x: X) = when (x) {
+    is X.A -> process(x)
+    is X.B -> process(x)
+}
+
+fun builder(c: suspend () -> Unit) {
+    c.startCoroutine(EmptyContinuation)
+}
+
+fun box(): String {
+    builder {
+        process(X.A())
+        process(X.B())
+    }
+    if (log != "from A;from B;") return log
+    return "OK"
+}

compiler/testData/codegen/box/coroutines/tailCallOptimizations/whenUnit.txt

@@ -0,0 +1,43 @@
+@kotlin.Metadata
+final class WhenUnitKt$box$1 {
+    inner class WhenUnitKt$box$1
+    method <init>(p0: kotlin.coroutines.experimental.Continuation): void
+    public final @org.jetbrains.annotations.NotNull method create(@org.jetbrains.annotations.NotNull p0: kotlin.coroutines.experimental.Continuation): kotlin.coroutines.experimental.Continuation
+    public final @org.jetbrains.annotations.Nullable method doResume(@org.jetbrains.annotations.Nullable p0: java.lang.Object, @org.jetbrains.annotations.Nullable p1: java.lang.Throwable): java.lang.Object
+    public final @org.jetbrains.annotations.Nullable method invoke(@org.jetbrains.annotations.NotNull p0: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public synthetic method invoke(p0: java.lang.Object): java.lang.Object
+}
+
+@kotlin.Metadata
+public final class WhenUnitKt {
+    private static @org.jetbrains.annotations.NotNull field log: java.lang.String
+    inner class WhenUnitKt$box$1
+    static method <clinit>(): void
+    public final static @org.jetbrains.annotations.NotNull method box(): java.lang.String
+    public final static method builder(@org.jetbrains.annotations.NotNull p0: kotlin.jvm.functions.Function1): void
+    public final static @org.jetbrains.annotations.NotNull method getLog(): java.lang.String
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X$A, @org.jetbrains.annotations.NotNull p1: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X$B, @org.jetbrains.annotations.NotNull p1: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X, @org.jetbrains.annotations.NotNull p1: kotlin.coroutines.experimental.Continuation): java.lang.Object
+    public final static method setLog(@org.jetbrains.annotations.NotNull p0: java.lang.String): void
+}
+
+@kotlin.Metadata
+public final class X$A {
+    inner class X$A
+    public method <init>(): void
+}
+
+@kotlin.Metadata
+public final class X$B {
+    inner class X$B
+    public method <init>(): void
+}
+
+@kotlin.Metadata
+public abstract class X {
+    inner class X$A
+    inner class X$B
+    private method <init>(): void
+    public synthetic method <init>(p0: kotlin.jvm.internal.DefaultConstructorMarker): void
+}

compiler/testData/codegen/bytecodeListing/tailcall/whenUnit.kt

@@ -0,0 +1,12 @@
+sealed class X {
+    class A : X()
+    class B : X()
+}
+
+suspend fun process(a: X.A) {}
+suspend fun process(b: X.B) {}
+
+suspend fun process(x: X) = when (x) {
+    is X.A -> process(x)
+    is X.B -> process(x)
+}

compiler/testData/codegen/bytecodeListing/tailcall/whenUnit.txt

@@ -0,0 +1,26 @@
+@kotlin.Metadata
+public final class WhenUnitKt {
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X$A, @org.jetbrains.annotations.Nullable p1: java.lang.Object): java.lang.Object
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X$B, @org.jetbrains.annotations.Nullable p1: java.lang.Object): java.lang.Object
+    public final static @org.jetbrains.annotations.Nullable method process(@org.jetbrains.annotations.NotNull p0: X, @org.jetbrains.annotations.Nullable p1: java.lang.Object): java.lang.Object
+}
+
+@kotlin.Metadata
+public final class X$A {
+    inner class X$A
+    public method <init>(): void
+}
+
+@kotlin.Metadata
+public final class X$B {
+    inner class X$B
+    public method <init>(): void
+}
+
+@kotlin.Metadata
+public abstract class X {
+    inner class X$A
+    inner class X$B
+    private method <init>(): void
+    public synthetic method <init>(p0: kotlin.jvm.internal.DefaultConstructorMarker): void
+}