vect: Allow unsigned-to-signed promotion in vect_look_through_possible_promotion [PR115707]

The function fails to figure out root definition if casts involves more than
two promotions with sign change as:

long a = (long)b;       // promotion cast
 -> int b = (int)c;     // promotion cast, sign change
   -> unsigned short c = ...;

For this case, the function thinks the 2nd cast has different sign as the 1st,
so stop looking through, while "unsigned short -> integer" is a nature sign
extension.

2024-08-05 Feng Xue <fxue@os.amperecomputing.com>

gcc/
	PR tree-optimization/115707
	* tree-vect-patterns.cc (vect_look_through_possible_promotion): Allow
	unsigned-to-signed promotion.