[Bug target/113838] regression of redundant load operation introduced by -fno-tree-forwprop introduce

["absoler at smail dot nju.edu.cn" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113838

--- Comment #5 from absoler at smail dot nju.edu.cn ---
(In reply to Andrew Pinski from comment #2)
> The difference from the gimple level IR:
> ```
>   _14 = g_26[5][3][0];
>   _15 = (int) _14;
>   _16 = _13 ^ _15;
>   g_51 = _16;
>   if (_13 != _15)
> ```
> 
> vs:
> ```
>   _14 = g_26[5][3][0];
>   _15 = (int) _14;
>   _16 = _13 ^ _15;
>   g_51 = _16;
>   if (_16 != 0)
>     goto <bb 4>; [50.00%]
>   else
>     goto <bb 3>; [50.00%]
> ```
> 
> 
> This is expected behavior even for the x86_64 target

The gimple ir has no problem, but `_13` is replaced with g_26[5][3][0] in the
follow-up process, this shouldn't be expected behavior.

We question this option because we found in an older version of gcc (10.2.0),
only the O2 option is needed to produce the same bad code, so we worry about
there's a hidden un-fixed problem and it's re-triggered by this option.

Besides, the bad binary code introduce more load operation than the source code
(without optimization), so we thought it's necessary to check it regardless of
which optimization is disabled.