[Bug target/96789] x264: sub4x4_dct() improves when vectorization is disabled

[linkw at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96789

--- Comment #13 from Kewen Lin <linkw at gcc dot gnu.org> ---
>   2) on Power, the conversion from unsigned char to unsigned short is nop
> conversion, when we counting scalar cost, it's counted, then add costs 32
> totally onto scalar cost. Meanwhile, the conversion from unsigned short to
> signed short should be counted but it's not (need to check why further). 

UH to SH conversion is true when calling vect_nop_conversion_p, so it's not
even put into the cost vector. 

tree_nop_conversion_p's comments saying:

/* Return true iff conversion from INNER_TYPE to OUTER_TYPE generates
   no instruction.  */

I may miss something here, but UH to SH conversion does need one explicit
extend instruction *extsh*, the precision/mode equality check looks wrong for
this conversion.