vect: Simplify epilogue reduction code

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".