Cleanup code around JERRY_UNREACHABLEs (#2342)

`JERRY_UNREACHABLE`s often signal code structure that could be
improved: they can usually either be rewritten to `JERRY_ASSERT`s
or eliminated by restructuring loops, `if`s or `#if`s. Roughly,
the only valid occurences are in default cases of `switch`es. And
even they can often be merged into non-default cases.

Moreover, it is dangerous to write meaningful code after
`JERRY_UNREACHABLE` because it pretends as if there was a way to
recover from an impossible situation.

This patch rewrites/eliminates `JERRY_UNREACHABLE`s where possible
and removes misleading code from after them.

JerryScript-DCO-1.0-Signed-off-by: Akos Kiss akiss@inf.u-szeged.hu

jerry-core/vm/vm-stack.c

@@ -65,8 +65,10 @@ vm_stack_context_abort (vm_frame_ctx_t *frame_ctx_p, /**< frame context */
       vm_stack_top_p -= PARSER_TRY_CONTEXT_STACK_ALLOCATION;
       break;
     }
-    case VM_CONTEXT_FOR_IN:
+    default:
     {
+      JERRY_ASSERT (VM_GET_CONTEXT_TYPE (vm_stack_top_p[-1]) == VM_CONTEXT_FOR_IN);
+
       ecma_collection_chunk_t *chunk_p;
       chunk_p = ECMA_GET_INTERNAL_VALUE_ANY_POINTER (ecma_collection_chunk_t, vm_stack_top_p[-2]);
       uint32_t index = vm_stack_top_p[-3];
@@ -96,11 +98,6 @@ vm_stack_context_abort (vm_frame_ctx_t *frame_ctx_p, /**< frame context */
       vm_stack_top_p -= PARSER_FOR_IN_CONTEXT_STACK_ALLOCATION;
       break;
     }
-    default:
-    {
-      JERRY_UNREACHABLE ();
-      break;
-    }
   }
 
   return vm_stack_top_p;