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/tailCallIntrinsics.kt

@@ -0,0 +1,18 @@
+// WITH_RUNTIME
+import kotlin.coroutines.experimental.*
+import kotlin.coroutines.experimental.intrinsics.*
+
+var p: Int = 5846814
+private suspend fun withoutInline() {
+    val c = { c: Continuation<Unit> ->
+        if (p > 52158) Unit else COROUTINE_SUSPENDED
+    }
+
+    return suspendCoroutineOrReturn(c)
+}
+
+private suspend fun withInline() {
+    return suspendCoroutineOrReturn { c ->
+        if (p > 52158) Unit else COROUTINE_SUSPENDED
+    }
+}