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/unreachable.kt

@@ -0,0 +1,118 @@
+suspend fun empty() {}
+suspend fun withoutReturn() {
+    empty()
+}
+
+suspend fun twoReturns() {
+    return empty()
+    return empty()
+}
+
+suspend fun notTailCall() {
+    empty()
+    return empty()
+    empty()
+}
+
+suspend fun lambdaAsParameter(c: suspend () -> Unit) {
+    c()
+}
+
+suspend fun lambdaAsParameterNotTailCall(c: suspend () -> Unit) {
+    c()
+    return c()
+    c()
+}
+
+suspend fun lambdaAsParameterReturn(c: suspend () -> Unit) {
+    return c()
+    c()
+}
+
+suspend fun returnsInt() = 42
+suspend fun callsIntNotTailCall() {
+    returnsInt()
+    return
+    empty()
+}
+
+suspend fun multipleExitPoints(b: Boolean) {
+    if (b) empty() else withoutReturn()
+    return
+    empty()
+}
+
+suspend fun multipleExitPointsNotTailCall(b: Boolean) {
+    if (b) empty() else returnsInt()
+    return
+    empty()
+}
+
+fun ordinary() = 1
+inline fun ordinaryInline() {
+    ordinary()
+}
+
+suspend fun multipleExitPointsWithOrdinaryInline(b: Boolean) {
+    if (b) empty() else ordinaryInline()
+    return
+    empty()
+}
+
+suspend fun multipleExitPointsWhen(i: Int) {
+    when (i) {
+        1 -> empty()
+        2 -> twoReturns()
+        3 -> withoutReturn()
+        else -> lambdaAsParameter {}
+    }
+    return
+    empty()
+}
+
+suspend fun <T> generic(): T = TODO()
+suspend fun useGenericReturningUnit() {
+    generic<Unit>()
+    return
+    empty()
+}
+
+class Generic<T> {
+    suspend fun foo(): T = TODO()
+}
+
+suspend fun useGenericClass(g: Generic<Unit>) {
+    g.foo()
+    return
+    empty()
+}
+
+suspend fun <T> genericInferType(c: () -> T): T = TODO()
+suspend fun useGenericInferType() {
+    genericInferType {}
+    return
+    empty()
+}
+
+suspend fun nullableUnit(): Unit? = null
+suspend fun useNullableUnit() {
+    nullableUnit()
+    return
+    empty()
+}
+
+suspend fun useRunRunRunRunRun() {
+    run {
+        run {
+            run {
+                run {
+                    run {
+                        empty()
+                    }
+                }
+            }
+        }
+    }
+    return
+    empty()
+}