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/ecma/base/ecma-property-hashmap.c

@@ -539,8 +539,6 @@ ecma_property_hashmap_find (ecma_property_hashmap_t *hashmap_p, /**< hashmap */
       JERRY_ASSERT (entry_index != start_entry_index);
 #endif /* !JERRY_NDEBUG */
     }
-
-    JERRY_UNREACHABLE ();
   }
 
   while (true)