[Bug tree-optimization/40073] Vector short/char shifts generate sub-optimal code

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40073

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to alalaw01 from comment #6)
> Same problem on AArch64 (at -O3). We've decided the shift is to be done on
> integers, widening the arguments and then truncating, before we hit the
> vectorizer:

This is needed for correctness due to c/c++ promotion rules. 

> 
>   int i;
>   short unsigned int _4;
>   int _5;
>   int _8;
>   short unsigned int _9;
>   int pretmp_18;
>   unsigned int ivtmp_21;
>   unsigned int ivtmp_22;
> 
>   <bb 2>:
>   pretmp_18 = (int) j_6(D);
> 
>   <bb 3>:
>   # i_14 = PHI <i_11(4), 0(2)>
>   # ivtmp_22 = PHI <ivtmp_21(4), 1024(2)>
>   _4 = b[i_14];
>   _5 = (int) _4;
>   _8 = _5 << pretmp_18;
>   _9 = (short unsigned int) _8;
>   a[i_14] = _9;
>   i_11 = i_14 + 1;
>   ivtmp_21 = ivtmp_22 - 1;
>   if (ivtmp_21 == 0)