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/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
+}