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/builtin-objects/ecma-builtin-helpers-date.c

@@ -740,8 +740,10 @@ ecma_date_to_string_format (ecma_number_t datetime_number, /**< datetime */
         number_length = 2;
         break;
       }
-      case LIT_CHAR_UPPERCASE_Z: /* Time zone seconds part. */
+      default:
       {
+        JERRY_ASSERT (*format_p == LIT_CHAR_UPPERCASE_Z); /* Time zone seconds part. */
+
         int32_t time_zone = (int32_t) ecma_date_local_time_zone (datetime_number);
 
         if (time_zone < 0)
@@ -753,11 +755,6 @@ ecma_date_to_string_format (ecma_number_t datetime_number, /**< datetime */
         number_length = 2;
         break;
       }
-      default:
-      {
-        JERRY_UNREACHABLE ();
-        break;
-      }
     }
 
     format_p++;