vect: Use an array to replace 3 relevant variables

vect: Use an array to replace 3 relevant variables

It's better to place 3 relevant independent variables into array, since we
have requirement to access them via an index in the following patch. At the
same time, this change may get some duplicated code be more compact.

2024-06-16 Feng Xue <fxue@os.amperecomputing.com>

gcc/
	* tree-vect-loop.cc (vect_transform_reduction): Replace vec_oprnds0/1/2
	with one new array variable vec_oprnds[3].