vect: Simplify epilogue reduction code
vect_create_epilog_for_reduction only handles two cases: single-loop
reductions and double reductions. “nested cycles” (i.e. reductions
in the inner loop when vectorising an outer loop) are handled elsewhere
and don't need a vector->scalar reduction.
The function had variables called nested_in_vect_loop and double_reduc
and asserted that nested_in_vect_loop implied double_reduc, but it
still had code to handle nested_in_vect_loop && !double_reduc.
This patch removes that and uses double_reduc everywhere.
gcc/
* tree-vect-loop.c (vect_create_epilog_for_reduction): Remove
nested_in_vect_loop and use double_reduc everywhere. Remove dead
assignment to "loop".