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/tryCatchTailCall.kt

@@ -0,0 +1,10 @@
+suspend fun catchException(): String {
+    try {
+        return suspendWithException()
+    }
+    catch(e: Exception) {
+        return e.message!!
+    }
+}
+
+suspend fun suspendWithException(): String = TODO()